The Level Design Book, an amazing free online resource.
This resource definitely deserves more attention, and, albeit being incomplete in some points, I've been finding to be extremely useful while designing my game, not just its levels. In particular the initial chapters about experience goals have changed my perspective on design.
Thanks so much for posting this! I have been looking everywhere for a good resource on level design! Seems like one of the least written about topics in gamedev. If you know if any more useful level design resources I’d love to know about them.