Please share any resources for high school electives. I'll update this OP as people post. Feel free to leave a personal review, ask questions, etc. Even if you see a program already listed, feel free to add your own personal review in the thread.
This listing is not an endorsement by GCM.
Psychology textbooks by David G. Meyers. These are not homeschool textbooks, unfortunately, but I have read several reviews that stated he writes textbooks that are truly engaging and enjoyable while still covering immense ground. He is a Christian (works at Hope College) but his textbooks are secular.
http://www.davidmyers.org/Brix?pageID=5
Introductory
Horticulture by Shry and Reiley.
Christian Light Publications has many books for a wide variety of electives-- automotive, technical drawing, bookkeeping, etc. While browsing, I noticed that they are often selling used, outdated versions for the same price as I would pay for a new, current version elsewhere. So this site might be a good place to get ideas but definitely shop around before purchasing.
https://www.clp.org/store/by_grade/17