I've been following
Charles' blog for a while now. He has some very cool design break downs and this guide is great as well.
One of the things he links to is the
Fundamentals of Design by Professor Alexander Slocum (He's an FRC mentor by the way). I found it about a month ago and have been going through the whole course. For anyone that didn't have formal mechanical engineering training it's a fantastic guide to design. I got to learn the names of a bunch things that I've sort of figured out over the years but never knew they had names.