I read the max happy book and timed it to start; just, after the 15 min mark. Happiness reverts back to your base happy at the next 15 min mark.
If I had read the repeat last book; this would not have occurred as I lost my 99k happiness before the next 15 min mark because I got a " system message saying my book ended, " which is fine but should not have removed my 99k happiness before the next 15 min mark.