Many folks who write stitch guides are affiliated with a shop and often teach there, so there are a goodly number of stitch guide writers who are using a stitch guide as class material, and there are quite a few shops that sell class material as stitch guides now. Robin King shows how she uses a stitch guide as a jumping off point for a lesson plan.
Seeing this reminded me of something I meant to mention. The difference between class materials and stitch guides is getting smaller, but there is still the issue of class guides being written to be taught in person by the writer. For example, links might have been provided to the purchaser to videos that were used in class but sometimes those are not available. There might have been handouts in class illustrating a point that never made it into the stitch guide, or the teacher might have spent extra time going over something that students found confusing, but never updated the guide with further information. I would suggest you find out if a mail order class guide has all the material that was provided in the actual classroom unless you are feeling particularly adventuresome. The teacher obviously won't be there with you, but if there are items that were used in class that aren't part of the mail order class guide, you will want to know before you buy!