Rough Weather is, according to the blurb (I was too lazy to actually count at SYKM), the 36th in the Spenser series by Robert B Parker. It has dropped to £2.99 at Kindle UK right now (April 4, 2021).
Promised Land is the fourth in Robert B. Parker’s Spenser series. Per SYKM (see link in OP above), it won the 1977 Edgar Award for Best Mystery.
It has dropped to $1.99 at Kindle US today: https://www.amazon.com/dp/B07XF7NX21
A Savage Place is the eighth in this classic series. It’s on sale for $2.99 right now at Kindle and Kobo US. Per eReaderIQ, it hasn’t been less than $5.93 since 2011, so I’d grab it now if you want it!
The Godwulf Manuscript is the one that started it all – the first in the Spenser series. And it’s $1.99 right now in the US, as part of today’s Kindle Daily Deal, and price-matched at Kobo US.
Silent Night is the last Spenser book that was written by Parker himself, and even it was completed by Helen Brann. Regardless, it’s on sale right now at Kindle US for $2.99, and it’s semi-long-term at £1.89 at Kindle UK.
Taming a Seahorse is the 13th in the Spenser series. It has dropped to $1.99 at Kindle and Kobo US. Some of Parker’s titles go on sale a lot at decent prices, but I haven’t seen this one much, if at all…
Promised Land is the fourth in Robert B. Parker’s Spenser series. Per SYKM (see link in OP above), it won the 1977 Edgar Award for Best Mystery.
It has dropped to $1.99 at Kindle US today: https://www.amazon.com/dp/B07XF7NX21
A Savage Place is the eighth in this classic series. It’s on sale for $2.99 right now at Kindle and Kobo US. Per eReaderIQ, it hasn’t been less than $5.93 since 2011, so I’d grab it now if you want it!
Kindle US: https://www.amazon.com/dp/B00309SCPS
Kobo US: https://www.kobo.com/us/en/ebook/a-savage-place
Small Vices, which is the 24th in the Spenser series by Robert B. Parker, is on sale right now for $2.99 at Kindle and Kobo US.
Kindle US: https://www.amazon.com/dp/B005F4CDCE
Kobo US: https://www.kobo.com/us/en/ebook/small-vices
Oh, and Promised Land, about two comments up, is down to $1.99 again at Kindle US. The link in the original comment is still good.
Looking for Rachel Wallace is the sixth in Parker’s Spenser series. It has dropped to $2.99 at Kindle and Kobo US.
Kindle US: https://www.amazon.com/dp/B0042JSO1M
Kobo US: https://www.kobo.com/us/en/ebook/looking-for-rachel-wallace-2
Paper Doll is the twentieth in the Spenser series. It has dropped to $2.99 at Kindle and Kobo US.
Kindle US: https://www.amazon.com/dp/B005F4CCY8
Kobo US: https://www.kobo.com/us/en/ebook/paper-doll-1
Stardust is Parker’s seventeenth Spenser title. It’s £2.99 right now at Kindle UK: https://www.amazon.co.uk/dp/B00CUE0AGI
Promised Land is $1.99 at Kindle US again today. The link in the comment way above is still good.
Mortal Stakes is the third Spenser title. It’s £2.99 right now at Kindle UK: https://www.amazon.co.uk/dp/B00BXFIZOY
Valediction is the eleventh in Parker’s Spenser series. It has dropped to $2.99 at Kindle and Kobo US.
Kindle US: https://www.amazon.com/dp/B002SVQD5K
Kobo US: https://www.kobo.com/us/en/ebook/valediction
Promised Land is £0.99 at Kindle UK right now: https://www.amazon.co.uk/dp/B07XF7NX21
The Widening Gyre is the tenth in Parker’s Spenser series. It’s $2.99 right now at Kindle and Kobo US.
Kindle US: https://www.amazon.com/dp/B002SVQD2S
Kobo US: https://www.kobo.com/us/en/ebook/the-widening-gyre
The Godwulf Manuscript is the one that started it all – the first in the Spenser series. And it’s $1.99 right now in the US, as part of today’s Kindle Daily Deal, and price-matched at Kobo US.
Kindle US: https://www.amazon.com/dp/B00309SD02
Kobo US: https://www.kobo.com/us/en/ebook/the-godwulf-manuscript-2
Walking Shadow is about halfway through the Spenser series. It has dropped to £2.99 at Kindle UK: https://www.amazon.co.uk/dp/B00CUE0AU4
Silent Night is the last Spenser book that was written by Parker himself, and even it was completed by Helen Brann. Regardless, it’s on sale right now at Kindle US for $2.99, and it’s semi-long-term at £1.89 at Kindle UK.
Kindle US: https://www.amazon.com/dp/B00E5UJAMA
Kindle UK: https://www.amazon.co.uk/dp/B00G8OBPLU
The Widening Gyre is £2.99 at Kindle UK right now: https://www.amazon.co.uk/dp/B00CUE0ABS
And Crimson Joy (#15) has also just now dropped to £2.99 at Kindle UK: https://www.amazon.co.uk/dp/B00COOHNBE
Silent Night is back for the holidays 😊. It’s £0.99 at Kindle UK right now, and the UK link in the comment three up from here is still good…
Pastime is the eighteenth in the Spenser series. It’s $2.99 right now in the US.
Kindle US: https://www.amazon.com/dp/B005F4CGA8
Kobo US: https://www.kobo.com/us/en/ebook/pastime
The Widening Gyre is $1.99 right now at Kindle and Kobo US.
Kindle US: https://www.amazon.com/dp/B002SVQD2S
Kobo US: https://www.kobo.com/us/en/ebook/the-widening-gyre
Aaaaand Promised Land is $1.99 again at Kindle US today: https://www.amazon.com/dp/B07XF7NX21
Taming a Seahorse is the 13th in the Spenser series. It has dropped to $1.99 at Kindle and Kobo US. Some of Parker’s titles go on sale a lot at decent prices, but I haven’t seen this one much, if at all…
Kindle US: https://www.amazon.com/dp/B002TXZS5I/
Kobo US: https://www.kobo.com/us/en/ebook/taming-a-seahorse
If you missed it back in April of last year, Promised Land is £1.89 at Kindle UK right now – a bit more, but still pretty good…
Kindle UK: https://www.amazon.co.uk/dp/B07XF7NX21