I'm trying to finish off my collection, too. I'm missing: 13. Enemy in the Fort, 14. Circle of Fire, 16. Whistler in the Dark, 17. Mystery at Chilkoot Pass, 21. Ghost Light on the Graveyard Shoal, and 22. Betrayal at Cross Creek.

I bought the other 16 at my local used bookstores, over the past few months, all for no more than $3 (and most of them "free" because we trade books a lot). They're almost all pretty easy to find. Check used book stores if you have any in your town.

If you have a daughter in the proper age range for the History Mysteries (10 and up, as stated on the back of the books), or for yourself, I'd definitely recommend the Girls of Many Lands books, if you haven't read those yet. I've loved every one that I've read, and really wish they'd write more books, even though the doll line failed.