Weight distribution on library ladder?
The library ladder kits I have assembled have been cut from 1X4 white oak for all parts, although other species of wood are fine too. The treads were strengthened by a piece of all thread stretched taught with a block to provide center support between the tread and all thread. This also holds it all together.

All the weight is transferred to the floor through the legs, the hooks at the top are for keeping the ladder to the wall, not for support.