How much is a text book from 1920's worth?

the clue of the broken locket / the message in the hollow oak

It depends on whether they are first edition or not. Also, is the dust jacket in good order, or is it without one? A first edition with intact dust jacket can bring thousands of dollars each book! Some sell for over $6,000 US. Check online, and if you can afford, get your books appraised. You will be pleasantly surprised. Good luck!