I wrote one on how to use an opensource testing tool for which there was lots of interest but not a lot of documentation at the time. That was quite popular for a while (almost 11 years ago).
Another book was on an emerging technique for collaborative analysis for software projects, that started becoming popular some time back in 2008-9 but was still not well understood.
Two more were on practical tips for how to work with user stories and how to come up with good testing ideas.
Interesting. If you don't mind saying, do you self-publish on sites like Gumroad, Leanpub, Softcover, on your own site, or both? If on your own site, you would need some system to accept payments, right?
I publish using leanpub and KDP (amazon kindle) for ebooks and using Lightning Source for paper books. I used Lulu 10 years ago but their customer service was appalling back then so I moved to LS, which gets the print book in almost all good online stores anyway.
I used to sell books on my own site as well until a few years back when EU created the stupid VAT on private electronic purchases that requires stores to adjust price according to the country of the buyer. I figured it's less hassle to just use leanpub for that.
Another book was on an emerging technique for collaborative analysis for software projects, that started becoming popular some time back in 2008-9 but was still not well understood.
Two more were on practical tips for how to work with user stories and how to come up with good testing ideas.