Word has it (from author and Catholic homeschool mom Melissa Wiley) that her prequels to the Little House series (currently published by Harper Collins) are going out of print, and will be re-issued only in an abridged format. We haven't read all of them yet (hasn't been a particularly good year for read-alouds) but have very much enjoyed the ones we have. I second Lissa's suggestion to pick these up while they're still available.
We were just filling out our Little House set last spring, after On Top of Concord Hill was no longer available through bookstores. Happily, we we found it still in stock at the Little House gift shop in Pepin, Wisconsin (last summer).
Don't miss out.