I've never had any luck re-purposing stuff like book cases. Its much easier and much better to just buy the plywood and 2x3s and build what you need in my experience. Most book shelves these days are built very flimsy and use particle board which sucks up water like a sponge and disintegrates.
For $400 you can buy 4 sheets of plywood and lots of 2x4s and 2x3' to make two solid 4x8' enclosures with stands. One sheet will be the bottom and the other sheet can be cut into 12" high walls.
Not to debate or argue, but I did finally buy 2 used solid wood bookshelves and lined the bottom with pond liner for my 2 desert tortoises. I was asking a friend to help me build something and he fell through. They are working for a nighttime enclosure, though I could probably leave them outside overnight because it’s been so hot out. Outside they are together in an enclosure where I added a wall to separate that leaves a space for them to get through. Maybe I should completely walk them off from each other. They are female (I’ve been told) siblings I’ve had since eggs. They are about 3 years old and growing so big I’m wondering if my counting age is off. Could I be overfeeding? The soil I’m using is so pure, I was worried about fertilizers, that plants, lettuce etc won’t grow so I’m buying stuff from organic store. Walking them off would give hem less space so maybe I should look for another space in our yard. I have seen one flip the other over. They sleep together sometimes. Thanks and good luck with yours!