It looks as if Sue Henry got back the rights to some of her Jessie Arnold/Alaska books, which have been re-pubbed by Speaking Volumes, which sounds like an audiobook publisher, but doesn't seem to be. And then a bunch of others still have e-books by HarperCollins. Although the two different publishers are scattered throughout the series, which is kind of odd - usually the early books are one publisher and the later books another. But in any case, HarperCollins have been slowly putting some of the books they are publishing on sale from time to time, and now it's the ninth book,
Cold Company, on sale in the US for $1.99.
Kindle US:
https://www.amazon.com/dp/B000OI0F8A
Kobo US:
https://www.kobo.com/us/en/ebook/cold-company