Too many libraries are closing which will make it more difficult for people to "challenge those in power", an ex-National Library for Wales chief says.
Andrew Green, the former head of the Aberystwyth-based library, also said councils had a statutory duty to provide an efficient library service.
It comes as figures released to BBC Wales' Newyddion 9 programme show nearly 30 libraries in Wales closed over the past five years.
About 14 others are at risk.
Of the 22 local authorities it contacted, all but three replied and gave the figures.
They also showed another 14 libraries have been transferred to the local community, thereby reducing the councils' responsibility for their upkeep.
Mr Green warned the closures - many due to council cutbacks - would impact on people without much money.
"A lot do use the libraries who don't have the resources to buy books for themselves or access to the internet," he said.
"Libraries are extremely important for gaining knowledge about the world, about discovering things, about educating yourself and, in the end, challenging people in power."
In the 19 councils that responded to Newyddion 9:
- 26 libraries have closed within the past five years
- 14 others have been transferred to the local community
- Another 14 branches could be in danger of closing
A motion calling on Cardiff council to think again about reducing library funding by £283,000 failed in City Hall on Thursday.
It means the future is uncertain for seven libraries in the city - Cathays, Roath, Rhydypennau, Rumney, Radyr, Rhiwbina and Whitchurch.