There are a few previously traditionally published mystery authors who sell their backlist through Smashwords. Per
this post, Smashwords is having its "Read an Ebook Week" sale, with discounts from 25% to 100%.
One nice thing is that Smashwords is DRM-free and you can download multiple formats with a single purchase. And one other nice thing is that they list the coupon code you need to get the discount right in the webpage for each book - although not in the author webpage, so you have to click through to each book to see how much (if any) discount is offered.
Most of these authors/titles cycle through various sales pretty often, but if you are following one or more of these, you may be able to fill in some missing titles.
Gillian Roberts has many titles, including most or all of her Amanda Pepper series, on sale for 25% - 50% off. There are some bundles that offer pretty good prices if you need all the titles in the bundle.
link:
https://www.smashwords.com/profile/view/gillianroberts
Sarah Shankman has several books, including a few titles in her Samantha Adams series, available mostly at 50% off.
link:
https://www.smashwords.com/profile/view/sarahshankman
Stephen Humphrey Bogart has both titles in his R.J. Brooks series at 50% off, and also a reasonably well reviewed (4 stars on Amazon) biography of his father (Humprey Bogart, of course

) at 50% off.
link:
https://www.smashwords.com/profile/v...humphreybogart
MK Wren has most or all of the titles in her Conan Flagg series at 25%, including some bundles.
link:
https://www.smashwords.com/profile/view/mkwren
Marilyn Todd has all (AFAIK) of the titles in her Claudia Seferius and High Priestess Iliona series on sale at 25% off, except for the first in each series, which are already priced lower than the rest.
link:
https://www.smashwords.com/profile/view/marilyntodd
Dorien Grey has several of the titles in the Dick Hardesty series on sale at 25% off.
link:
https://www.smashwords.com/profile/view/doriengrey3
Kaye George does not have the Imogene Duckworthy series on sale, but does have a non-series title (possibly the first in a new series),
Death in the Time of Ice, on sale at 50% off. Per the blurb,
Death in the Time of Ice was nominated for Best Historical Mystery in the 2014 Agatha Awards.
link:
https://www.smashwords.com/books/view/549952