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.