An exercise guide, a James Patterson best-seller and true-crime books were among those most borrowed at a Welsh prison.
Of the 10,000 titles available to HMP Cardiff prisoners, Essential Abs was the most popular book since 2012.
Data released by the Ministry of Justice (MoJ) showed detective story Merry Christmas, Alex Cross was the most loaned novel.
The MoJ said improving prisoner literacy and education is "essential".
The figures, released to the BBC after a Freedom of Information request, showed Essential Abs was borrowed 40 times by 37 inmates at the Category B prison in 2013 and again in 2014.
The Alex Cross novel - the 19th in James Patterson's series which follows the eponymous investigator - was borrowed 34 times in 2013 and 18 times in 2014.
Chasing Killers: Three Decades of Cracking Crime in the UK's Murder Capital, a factual book about Glasgow's underworld, was borrowed 21 times in 2013 but dropped out of the top five the following year.
Figures for Wales' other prisons were not available as the jails and their associated library services do not record the frequency of loans.
An MoJ spokesman said: "Governors have the discretion to withhold any books which they deem inappropriate.
"Prison should offer offenders the chance to get the skills and qualifications they need to make a success of life on the outside.
"Providing proper education opportunities and giving prisoners the tools they need to improve their literacy is essential."
Prison libraries do not keep books on home brewing, electrics, weapons or bomb making or titles that support or promote extremist views.
Michael Gove lifted restrictions on the number of books prisoners can have in July, shortly after becoming justice secretary.
A limit of 12 books per cell was removed, while relatives and friends can now send books to inmates directly, contrary to previous limitations.
- Essential Abs - 40 loans in 2013, 40 loans in 2014
- Merry Christmas, Alex Cross - 34 loans in 2013, 18 loans in 2014
- Chasing Killers: Three Decades of Cracking Crime in the UK's Murder Capital - 21 loans in 2013
- Live by the Gun, Die by the Gun - 20 loans in 2014
- Chopper - 19 loans in 2013
- Crimes that Shocked the World: The Most Chilling True-life Stories from the Last 40 Years - 18 loans in 2013
- Crack House: The Incredible True Story of the Man Who Took on London's Crack Gangs and Won - 16 loans in 2014
- Gangs - 15 loans in 2014
The Howard League for Penal Reform has previously called for improved access to reading material for prisoners.
Andrew Neilson, director of campaigns, said: "We should be doing everything we can to encourage reading in prisons.
"Reading goes hand-in-hand with education and rehabilitation, and research shows that informal learning reduces reoffending."