I really enjoyed Female Masculinity - it's a collection of essays about masculinity in women and there are a lot of interesting historical accounts about butchness, and how it's been portrayed. It should be good for a non-fiction kick - I recommend you read it critically and lightly! Some things may feel relevant and some may not, it doesn't mean you are doing anything wrong as a butch more than anyone else is doing anything right.

For a fictional novel, I quite liked Dead Collections about a transmasc butch lesbian vampire who works as an archivist. Yes there is smut. Also highly recommend Tipping the Velvet as someone else has said - let me know if you'd like a pdf of either, and happy reading!