A fellow blogger, Melissa Wiley, wrote two additional series in the Little House chronicles. The books she wrote are now out of print but the publisher has released abridged versions (because in the world of Bible-length Harry Potter books, a 200 page book is just too much). Amazon is selling these abridged versions but isn't mentioning that they are abridged. The abridged books have photos on the covers instead of drawings.
If you liked the Little House series, you'll want to get the original books by Melissa as well. You can still find them used in various places.