Almost 8,000 jobs in UK libraries have disappeared in six years, about a quarter of the overall total, an investigation by the BBC has revealed.
Over the same period, some 15,500 volunteers have been recruited and 343 libraries have closed, leading to fears over the future of the profession.
Children's author Alan Gibbons said the public library service faced the "greatest crisis in its history".
The government said it funded the roll-out of wi-fi to help libraries adapt.
The BBC has compiled data from 207 authorities responsible for running libraries through the Freedom of Information Act. Our analysis shows:
- Some 343 libraries closed. Of those, 132 were mobile services, while 207 were based in buildings (and there were four others, such as home delivery services)
- The number of closures in England is higher than the government's official estimate of 110 buildings shut
- A further 111 closures are planned this year
- The number of paid staff in libraries fell from 31,977 in 2010 to 24,044 now, a drop of 7,933 (25%) for the 182 library authorities that provided comparable data
- A further 174 libraries have been transferred to community groups, while 50 have been handed to external organisations to run. In some areas, such as Lincolnshire and Surrey, the move has led to legal challenges and protests from residents.
Mr Gibbons, who wrote Blue Peter Book Award winner Shadow of the Minotaur, said: "Opening hours are slashed, book stocks reduced.
"Volunteers are no longer people who supplement full time staff, but their replacements. This constitutes the hollowing out of the service. We are in dangerous territory."
Philip Pullman, the author of the fantasy trilogy His Dark Materials, said the library service should not rely on volunteers.
"It is exploiting people's goodness and willingness to work and so on," he told BBC Radio Oxford.
"I am in favour of volunteering but relying on volunteers to provide a service that ought to be statutory is not a good policy. What next? Are we going to rely on volunteer teachers because we can't find new teachers because all the staffing levels in schools are going down?"
There are four areas - Sefton in Merseyside; Brent in north London, Stoke-on-Trent and Sunderland - where more than half the libraries have closed since 2010, either buildings, mobile or both.
Librarian Ian Anstice, who runs the Public Libraries News website in his spare time, said the cuts were "without precedent".
He said: "Councils learnt early on how unpopular simply closing libraries is so they have had to cut the vital service in other, less obvious ways.
"It can come across in many forms: reduced opening hours, reduced book fund, reduced maintenance and reduced staffing.
"In all its incarnations, it is harmful to the service, creating the risk that once-loyal users of libraries will come away disappointed and stop using them.
"Our public library system used to be envy of the world. Now it is used as a cautionary tale that librarians use worldwide to scare their colleagues."
Cuts to library services were deeper in England than the rest of the UK.
- In Scotland, there were 3,515 paid library jobs in 2010 and 3,416 in 2015, a drop of 99 (3%)
- In Wales, there were 1,241 jobs in 2010 and 979 today, a fall of 21%
- In Northern Ireland, the paid jobs fell from 922 to 719, a reduction of 203 (22%).
"Scotland has, until now, been faring better than south of the border for libraries, but I'm not sure it's going to be maintained," Professor Peter Reid, who oversees the Department of Information Management at Robert Gordon University in Aberdeen, said.
In England, the largest percentage drop in library workers was in Harrow in north-west London. In 2010, the council paid 164 people but today there are 60 employees, employed by an external provider, which now runs the service.
The largest rise in volunteers was in Hampshire, where 1,498 people are assisting the library service, compared with 567 at the beginning of the decade. Over the same period, paid staff in the county's libraries dropped from 760 to 525.
The East Midlands has lost 36% of its jobs in libraries, the biggest percentage drop in the UK. There were 1,905 paid posts in 2010 compared with 1,213 now.
Some authorities say changes were inevitable to the library service. Volunteers perform a range of tasks, from reading clubs to stacking shelves, and their use varies from library to library, they argue.
Councillor Andrew Gibson, from Hampshire County Council, said: "Our volunteers are enthusiastic and knowledgeable, and are greatly appreciated by library customers. Volunteers have helped libraries to increase their opening hours."
He said 58% of people responding to a recent residents' survey agreed with increasing the number of trained volunteers.
Harrow's Councillor Sue Anderson said: "We are faced with government cuts of £83m and like other local authorities we need to make the most from our reduced budget.
"Yes, we have needed to close four of our most under-used libraries (and merge two others), and some staff have left. But the model for libraries that we have now is not just sustainable, it offers a bright future for public space and learning."
BBC News also analysed how library habits have changed. Through analysing Chartered Institute of Public Finance and Accountancy reports from 2012 to 2014, we found:
- The amount of time spent on computers in public libraries has fallen across more than two-thirds of England
- The number of books borrowed from libraries fell in almost every area
- Councils in the North East and North West of England make up nine of the 10 areas recording the biggest fall, along with the London borough of Camden
- In Doncaster, where just five libraries are still run by the city council and 19 entrusted to community groups, there were 629,000 book "issues" in 2014, compared with 1.2 million two years earlier.
Dr Briony Birdi, who has an MA in librarianship, said: "It's not a happy picture, but there are still lots of libraries left.
"The danger is we predict the end of libraries, and it's not the end of libraries.
"We would be lying if we said [library staff] are not affected by what's going on. The way that funding has been repeatedly cut has been the most demoralising.
"I don't want people to think that libraries are over, they will just look different and I think we do accept that."
Elizabeth Elford, of the Society of Chief Librarians, said: "I think inevitably there will be fewer public libraries when we come out the other side, but they will be better and more innovative... we are really trying to be attuned to the needs of customers, above and beyond books."
Councillor Ian Stephens, who chairs the Local Government Association's Culture, Tourism and Sport Board, said: "It's testament to how much people value their libraries that so many have volunteered to help keep them open.
"Councils are doing all they can to keep libraries open and don't take decisions to close them lightly. Each council will do what works best for their communities depending on local circumstances."
A spokesman for the Department for Culture, Media and Sport said: "Libraries are cornerstones of their communities and are part of the fabric of our society, so it's vital they continue to innovate in order to meet the changing demands of those they serve.
"Government is helping libraries to modernise by funding a wi-fi roll-out across England that has benefitted more than 1,000 libraries and increasing access to digital services and e-lending.
"The Libraries Taskforce is also consulting on a new vision for public libraries that will help reinvigorate the service and ensure they remain relevant to local communities."
A spokesman for the Department of Culture Arts and Leisure Northern Ireland said it regarded libraries as a "key front line service", and had provided additional funding to protect them from the impact of budget cuts.
A Scottish government spokeswoman said: "We place great importance on public libraries and believe everyone should have access to library services."
Welsh Assembly Deputy Culture Minister Ken Skates said: "We took the decision during the worst of austerity to better protect budgets for Welsh councils and local libraries than happened in England."
Reporting team: Daniel Wainwright, Paul Bradshaw, Pete Sherlock, Antia Geada
Following publication three councils provided further information to that contained in their original FOI responses.
Derbyshire County Council has closed eight of its 10 mobile libraries while Gloucestershire County Council closed three and Lambeth closed two.