Self-pub, through Leanpub and eventually Kindle, Createspace, and iBooks, I'm up to around 100-200 sales/month (ASP ~$10, net ~$7), mostly by keeping my book updated with the latest version of the software I write about (so my book is the only one that's constantly relevant, unlike the ones from the major publishers). I spend maybe 2-4 hours/month updating the book, running tests and fixing things.