Oh god. I do this so much, not just with books, but also with other kinds of media, and even programming projects... Sometimes I even read like 90% of a book and then for some reason start reading a completely different one and only finish the book months later.
I switched to audiobooks. I just don't have the time to read like I used to. Originally got started on them by accident, but they've grown on me; considerably.
Why not? Granted I only get through 1 or 2 books a month, but it's a minuscule part of my budget. I can't say I've looked into it, but I assume it's also the best way to actually support the author.