The first four titles in the Li Yan/Margaret Campbell "China" series by Peter May have dropped to $2.99 each at Kindle and Kobo US, possibly in honor of the fifth and sixth titles coming out as e-books in the US (available as pre-orders now), although they've been available in the UK for a while. I've read and enjoyed some of May's other books, but have not read anything in this series.
Before you buy, note that the first title,
Firemaker, has been on sale before, and even free in a few places, so check carefully before you buy. Titles in this series are normally $9.99 in the US, which is, IMO, absurd for very old (early 2000's) backlist, and this is the first time I've seen any of them on sale, except for
Firemaker.
Firemaker, #1
Kindle US:
https://www.amazon.com/gp/product/B076ZKPCJK/
Kobo US:
https://www.kobo.com/us/en/ebook/the-firemaker-5
The Fourth Sacrifice, #2
Kindle US:
https://www.amazon.com/gp/product/B0763KCNBZ/
Kobo US:
https://www.kobo.com/us/en/ebook/the-fourth-sacrifice-5
The Killing Room, #3
Kindle US:
https://www.amazon.com/gp/product/B07C24TXV6/
Kobo US:
https://www.kobo.com/us/en/ebook/the-killing-room-16
Snakehead, #4
Kindle US:
https://www.amazon.com/gp/product/B07D6LMMVC/
Kobo US:
https://www.kobo.com/us/en/ebook/snakehead-19