Context
I intend to build a self-standing bookshelf using 18mm-thick, CNC-router-cut plywood. The bookshelf is made of nested planks, and entirely fits a 35cm-deep recess in my wall.
Following pictures display first a sketch of the global design, and second, how I intend to nest planks (click on picture to see them full size):
1/ 
2/
Problem:
Although I don't have any children and don't really expect some at my home during the lifespan of the bookshelf, I kind of fear that it could topple frontwards. I am not allowed to secure it using screws in the wall.
Question:
Should I consider tilting either the supports (picture A: α<90°), or the shelves (picture B: β>90°) — or even both — backward by a few degrees to prevent the bookshelf to topple? If yes, which solution is the best, and what angle would be advisable?
A/
B/
Question:
Best Answer
Use Screws Anyway
If it was me, I would quietly ignore "I am not allowed to secure it using screws in the wall." We are talking about a couple of screws here, which would leave ~ 1/4" holes, not cutting out large sections of the wall.
Assuming it is drywall or plaster over studs, as opposed to a brick wall, I would put in two screws (~ 2" long) through brackets on the top shelf positioned so that the screws go into studs. The patching needed when you leave, if any, is minimal - really would get taken care of trivially as part of a routine "get ready for a new tenant paint job". You'll sleep much better.