By Katherine Sellgren / BBC News
Inmates are allowed 12 books in their cell but new privileges regulations, introduced last year, stopped them receiving parcels, including books.
The Prisoners' Education Trust (PET) is urging ministers to improve access to books and materials to assist learning.
Prisons minister Andrew Selous said he was "committed to improving education".
The Ministry of Justice said prisoners had access to libraries and could ask to have books ordered in.
In a report, Brain Cells: Listening to Prisoner Learners, the PET said the restriction on the number of books prisoners could have in their cells was "inadvertently impacting on higher level and distance learners who need a variety of sometimes specialist books".
The report went on: "We recommend this is reviewed urgently to ensure that it does not inadvertently inhibit learning, and encourage prison governors to use their discretion in the meantime.
"This is especially important as currently staff shortages are making it increasingly difficult for prisoners to access prison libraries."
The trust said the restriction was affecting prisoners' "ability to study and hence impacting on their rehabilitation and risk of reoffending".