Reddit mentions of Wooden Puzzles: 20 Handmade Puzzles and Brain Teasers

There's a lot out there and it depends what sort of puzzles you want to make.

My favorites are the Burr Puzzles, especially the modern variants that have proliferated in the 21st century

Ishino's Site has hundreds of such designs - I believe the point of the site is that they are free for personal use - finding the simpler designs will be the hard part, but you can search by shape to keep things simpler. Designers to check out: Osamori Yamamoto, Gregory Benedetti, Jos Bergmans, Bill Cutler, Stephan Chomine, William Hu, Tom Jolly, Jeff Namkung....loads more, but I think all of these guys have a few relatively simple designs that are still worthwhile puzzles

Brian Menold's book has some great designs with step-by-step instructions. Little Kenny and Bundle of Sticks Jr. from this book are quite nice.

There's also the Charlie Self, Tom Lensch book

There are also the older books by E. M. Wyatt and Stewart Coffin that have puzzle designs like basic 6-piece burrs

Woodworking sites and magazines often have articles on puzzle-making

If you're more interested in making other types of puzzle, I'm sure you can find plenty of things on the web