Hold on Tight bookshelf. A brightly-coloured aluminum cube and oversized wing nut serve as a sliding bookend which can be adjusted and secured wherever it is needed, whether your book collection is growing or shrinking.
LR window seat inspiration--but with thick cushion. (Note how backrest plane and then seat plane expand overall width without having to thicken the wall.) Napa Valley House by Steven Harris Architecture (Architect: Eliot Lee)