Self-Defense is the 9th in the Alex Delaware series by Jonathan Kellerman. It has dropped to $2.99 at Kindle and Kobo US.
Additional note: For me, at least, this was eligible for the apparently-newly-renewed Kindle $5 off on some Penguin/Random House titles, so it was free for me. You can see more info on this very odd credit in this thread:
https://www.mobileread.com/forums/sh...d.php?t=321100
Kindle US:
https://www.amazon.com/dp/B000FBFN8S
Kobo US:
https://www.kobo.com/us/en/ebook/self-defense