Back when I was a student, the library used to gave away outdated text books for free. I used to take home old hardbacks and doodle on the pages, guilt-free (seeing as I was saving the books from a rubbish-bin death).
The following sketch was done in a book called ‘Recent Experiments in Psychology’, published in 1938. The particular pages discuss ‘the duplicity theory of vision’, and that’s where I got the idea for my drawing.
I’ve had this particular book for several years now, and I’m slowly filling it with my sketches.