A magus who prepares a spell using the class feature knowledge pool can copy that prepared spell into a spellbook—probably
That is, general consensus seems to be a tentative Yes in this 2011 Paizo messageboard thread. Developer Jason Bulmahn solicited opinions during the magus playtest as to whether this should be considered a bug or feature in this 2010 Paizo messageboard thread.
There is some discussion whether the magus technically understands the spell—a necessity when copying a spell from another caster’s spellbook or from a scroll—but there’s no allowance made for not understanding a spell that’s already prepared, the assumption likely being if it’s in your head, you understand it.
No one points to anything horribly unbalanced about this quirk, but several do voice that allowing the magus to do this makes them vaguely uncomfortable, but only insofar as such a process goes unmentioned in the magus’s description.