In my opinion, a caster's "spell book" is more of a collection of notes and observations and hints and so forth, rather than an actual spell (that would be a scroll). So, I wouldn't allow learning between level gains, and certainly not in the middle of a dungeon.
Once a character in Novus learns a spell, he knows it forever, so there is not really any need to copy spells into a spell book. Like I said, I envision it more as a book of notes and research material for spells that have already been learned (thus, while a caster's book can be used to learn spells, it takes research and time and other sources).