What do you do when you start a book you don't enjoy?

Sigh. I did the unthinkable today. I got more than halfway through a book and gave up. To be honest this is a new experience for me. To not finish a book I think is an insult to the author and even when I'm not enjoying a story much I always persevere in the hope that I'll have some sort of epiphany and finally 'get it'. Sadly this was not the case and I couldn't bring myself to read the final 100 pages.
The most confusing thing was that I'd read several books by this author in the past and e...