If you use platforms like Gumroad/Leanpub, you can use pay-what-you-want model. That is, give away the book for free but users can pay if they want. You could a

If you use platforms like Gumroad/Leanpub, you can use pay-what-you-want model. That is, give away the book for free but users can pay if they want. You could also have a free online version but charge for PDF/EPUB versions. I do a mix of both. Whenever I publish a new book, I give them away for free for a week or two, but I still get paid sales as well. Recently I started adding discounted prices for my earlier books along with new releases, so that's another option if you plan to write more books.