Reading the classics in the clink just got a lot harder.

Books sent to prisoners in three state lock-ups will have to come from pre-approved internet vendors under a new state pilot program.

The program, which went into effect last month and applies to the Greene, Green Haven and Taconic correctional facilities, will limit the numbers of books to which inmates have access — prompting an outcry from inmate-rights groups.

“Taking books away from people is unconscionable and seems counterproductive if the idea is to educate and decrease recidivism,” said Lisa Davidson, a spokeswoman for the nonprofit Books Through Bars.