Some prisoners that are in low custody classification may actually just be housed in a dorm style prison room with bunks spread out through the room where many prisoners spend their time in.
In higher security prisons the prisoners are usually locked down at night and then in the morning the cell is opened and if they are trusted and can get along with other inmates they can usually roam about in the day room of the prison and they do not have to spend all day in the prison cell.
For maximum custody prison inmates they usually have to stay in their prison cell all day or until they can be moved about the prison with handcuffs on and a guard escorting them.
Some prisoners also have jobs that they are required to do around the prison such as cooking, cleaning and even working in the prison automotive shop keeping prison vehicles working.
If the prisoner is in solitary confinement which is not a good thing then they stay in the prison cell for 24 hours per day and sometimes they get let out on some days to go into an outdoor rec cage for 1 hour per day.
Some prisoners in solitary confinement are allowed books and some are not allowed books or anything else but the essentials.
If you're in solitary confinement and have had all your privileges taken away from you then no you won't be allowed books in solitary confinement or anything else.
But if you still have your privileges then yes you can have books to read while in solitary confinement which is a good thing because even a book could help the prisoner from going even more insane.
I go insane at times just being stuck inside my house on cold winter days when the weather is very nasty outside and so I could not imagine being locked in a solitary confinement cell for 23 or 24 hours per day for months on end or even sometimes decades.
There have been some prisoners who lived in solitary confinement for 20 years and longer without being let out except for maybe 1 hour per day and into a rec cage.