Hi,
This is something I've also been looking into recently and finding a good bookcase (depth wise) that is in my price range has been quite difficult. I personally feel that as long as your hamsters get out of cage time in a safe play area regularly the size of the bookcase cages should be okay.
Theres some links here which may be helpful for you to read through
DIY Cage - Ikea Regissor Bookcase
IKEA Billy bookcase conversion - upright
The pictures dont show anymore on this thread but the bookcase this user used was only 30cm deep which is only 2cm more than the billy
Project: Syrian cages
For hanging things from the ceiling perhaps you could use these
80mm Stainless Steel Wood Screw Cup Hook - GS Products
im not sure how well they'd work but thought maybe they'd be useful - you'd have to be sure the hamster cant reach them though
Alot of members who have tanks use industrial strength Velcro for their water bottles perhaps this could be an option for the bookcase? Or a free standing water bottle holder could work too
I reckon it could work out well as long as you protect the wood by sealing it and ensure theres enough ventilation. I hope this helped and i'll be interested to see what others say on this thread too as a bookcase diy is high on my future cages list