The "official" O'Reilly books are pretty good, Learning OpenCV by Kaehler and Bradski (I say official because Gary Bradski was one of the original maintainers). Make sure you get the new one for OpenCV 3. They cover a lot of the algorithms behind the code which is nice.
There's also https://www.pyimagesearch.com/ which is great. It's ultimately a hook to get you to buy the course, and I think the teaching style may put off some people, but everything is thorouhgly explained and there's a lot of cool mini projects.
There's also https://www.pyimagesearch.com/ which is great. It's ultimately a hook to get you to buy the course, and I think the teaching style may put off some people, but everything is thorouhgly explained and there's a lot of cool mini projects.