"Self-help" or "How to" books stand a better chance of selling.
Essentially the author is writing about something he/she has expertise in and there are bound to be people who would buy a book to learn more on that specific subject. You're selling knowledge!
Self-help books also easily lend themselves to having speaking engagements and conducting seminar/workshop classes where more books can be sold. Over the last couple of years I've offered a couple of continuing education classes through my school district a few times a year where my book is packaged as a material fee for the class.
Each student pays me the full amount $10 for the book in class.
Naturally books sold during seminars/workshops and the like aren't counted when it comes to "best seller" list rankings because the author is supplying the books as opposed to having the reader order them through Amazon or wherever. Nevertheless having the option to sell books in person still can put money in one's wallet. The beautiful thing about the continuing education classes is the author simply writes a course description and the school district prints up the catalogs and mails them to current and past students. In my case they also pay me an hourly fee to teach a class around the principle of my book. These classes are a "one and done" deal lasting no more than 2 hours. I like doing classes on Saturdays from 10AM- Noon.
Every self-published author could write a proposal to their school district or local community colleges offering to teach a class on "how to self-publish a book". Naturally you'd have some copies of your own book to pass around for them to look at. Odds are some students will buy a copy from you.
You could also log onto Meetup.com and look up book clubs and other groups that may be related to your book. Check with the organizer to see if they might be interested in having a guest speaker/author for a meeting. Always bring copies of your book.