Back Story is, per the blurb, the 30th in the Spenser series by Robert B. Parker. These go on sale a little more often than the Travis McGee books, but for most of them, not a whole lot more often. So again, grab it if you want it. It's $1.99 at Kindle and Kobo US.
Kindle US:
https://www.amazon.com/dp/B000OIZSJQ
Kobo US:
https://www.kobo.com/us/en/ebook/back-story