Gumroad, Leanpub, Softcover.io . There are other options. If you're looking for more direct ways to promote to that target audience you mention, social media

Gumroad, Leanpub, Softcover.io . There are other options. If you're looking for more direct ways to promote to that target audience you mention, social media sites could be one way, e.g. Twitter and Facebook. Not too sure about the ways to do it on Facebook - maybe there are groups for those topics that you can join, and then post about your book there now and then. On Twitter, you can post about your book with appropriate hashtags. Also search using http://hn.algolia.com http://hn.algolia.com for topics like self-publishing, ebooks, etc.