I owned a stack of books when Pook was born; I had been a preschool teacher. But instead of putting all the books out on his bookshelves at once, I introduced a new book about once a week. He'd get a new book and we'd read it to death for a week. He knew it inside and out and loved each book separately. He knew which books had been given to him as gifts and he (and I) had memories of the introduction of each one.
With Bug, I simply shifted all the picture books from Pook's shelf to his. Pook still read them some, so I couldn't pack them up. (I used to pay Pook 5c to read a board book aloud to Bug!) Instead Bug has always had a full bookshelf and all the books have been equals to him. My favorites are not his. I don't know if he has any favorite picture books.
My purging has moved on to books. Last summer I ruthlessly emptied the house of board books. It was a bit sad, but they have not been missed. (And I've saved the good ones anyway.) It made space for the early readers that Bug was starting to use.
But now Bug is well into chapter books. We still read picture books but we don't need them. I just removed a pile which I put in storage, but left accessible. Then I brought a dozen downstairs. I will try a new idea. When they come home from school, I will read a picture book to them both, for old time's sake, and then we will pack it up to put it away. One book at a time (for the next 4000 days).
I hate to push them to grow up too fast. I want the happy balance between getting big kid stuff and keeping little kid stuff. I also want it all to fit on the shelves, which it does not.
Our other book problem is ownership. As Pook got older, he sometimes used his own money on books. I know he saved for a long time to buy hardback copies of a few. So, should these now go to Bug? I can't keep up with who owns what book. I don't want fights over who is allowed to own which. I know Pook is attached to books he paid good money to buy, and someday when he's older he should have the right to sell them or keep them. But for now, Bug needs to read them.
Bug is overwhelmed right now by the number of books which we own at his reading level. He can read most any simple chapter book and wants to devour them all. I want to hand out the books slowly so he can enjoy them one at a time. Removing Pook's books from the shelves so that I can slowly give them to Bug seems unfair, but it may be what needs to be done.