I think that Minerva McGonagall was fully aware that the marauders were trying to become animagi and she may or may not have carefully placed some useful reference books, some of which may or may not have been her own personal copies, in the library where she knew they would find them.
James coming back to the common room late to find Remus asleep on a chair with an open book on his lap. James sighing and piggybacking the sleepy friend up to bed.
Sirius has fallen asleep on the train again. We’re going to draw willies on his face with Permanent Ink.