That is, likewise, a consequence of the unfortunate app re-launches (compare
GitHub #18). Normally, if you go from Library to your book, then pressing the "Home" button in the book returns you to the
Library,
not the Home screen. However, if a Marvin re-launch interferes in between (and iOS forces those re-launches every now and then), then it "inserts" the Home screen into your work-flow, so to say.
The "inconsistency" of the Home button inside the book -- that it may return you to the Library on one occasion, and to the Home screen on another occasion -- has been commented upon previously by other users, who were confused by this
seeming unpredictability.