I never stole the book from the monastery when I played trough it. But you can re enter the monastery again later while doing the restless sprit quest. Enter through the way you escaped, cause that door you ran out the 1st time, will still be wide open (that’s if you saved Pious/let him go/sent him to prssion). go in at night at about 1 am. The monks are supposed to all be asleep but instead they are all sitting eating in the dining hall. Hot foot it to the library pick the “hard lock” to get into the restricted area and then pick the lock on the trunk to get the key that opens the locked cabinet in the library.
If for some reason the book is not there (because you stole it earlier or it’s bugged and not showing up on the shelf). Head down to the dining hall and look fo one of the novice monks in white (think he is near the top left of the table near the Windows ) croach down and stealth mug him - don’t kill him) he will have a second copy of the book in his inventory. Nab the book and hi-tale it out of there the way you came. The monks will run around in confusion after you mug the novice and gaurds will come, just be quick and head out the way you came in. There is a steep cliff and the gaurds can’t follow you. (This won’t affect your reputation in anyway). Then take the book now called Necronomicon11 back to the apothecary in rattay and the objective will complete and you can carry on with the quest.
Note: because all the monks were all eating all the time, till you mug the novice (cause I think it’s bugged lol). You have free rein of the monastery. I took a shit load of lock picks in with me and wore my darkest slealthiest clothing and ransacked the whole joint. Took a shit load of books aswell 
Just save after you loot the moastary and/or you could not find the book before going and mugging the novice monk, cause you may mug the wrong one lol, in that cause just re load and mug the novices till you find the rite one that has the second version of the book.
Sorry for the edits. Typing on an iPad, auto correct sux and this forum is not so mobile browser friendly 