Add books from leadership workshop to handbook

These are the books from https://docs.google.com/presentation/d/1P5j481lHU34PX2mwIJIo9hjrc_eDH9Yad0zDjF8eGeU/edit#slide=id.g57c5a9b892_0_43 that weren't already on the leadership page.

I had a small doubt about the line 'books in this section can be expensed', because books can be expensed anyway. But according to 24b14385 that line was just to reinforce that point, not because the books here have a different status.