thanks for your comments, Facet.
Having platforms (the books) that appear to recede in space does raise the question of exactly what plane the character is on; which edge of the book?
The idea is to mix both interpretation in a way that will not trick the player: you can walk all over the book top, and the blue line can be jumped through. only the "front side" of the book is really solid (and thus cannot be walked or jumped through). This allows me to use stacks of books either as ladders, walls or more subtle hidden tunnels to secret rooms
I really like the book fort architecture of the background , but having the books unilaterally larger (and significantly so) than those of the foreground invites a sense of dislocation of scale. I think the previous, plainer backgrounds felt more appropriately intimate and 'zoomed in'. Personally I'd apply that (very nice) idea to the foreground platforms (the current foundation of woodblocks seem uninspired and not particulary evocative of a classroom). BG books could look good as well but I'd keep them at the same scale or smaller.
That's noted, and meets comments I got from non-artist friends. "intimate" is the word. I do love that on the plain background, too, although I can't help thinking it's not ambitious enough. I suppose I just need to walk my way through it and find more ways to make this wooden BG interesting and sufficiently diverse.
And I'll definitely go for more type of books and less wooden blocks in the foreground ... just enough to lay out "shelves" on which the books stand (the current level is more the school's library than a giant classroom, btw).