as far as I know there is nothing currently out there similar to A Color Guide to Corn Snakes (a great book!). I did find this book when I did a search but no idea about the price or quality:
http://www.designermorphs.com/
My best advise is websites, there are quite a few that discuss the various boa mophs, a section here on KS as well. I also look at classifieds or breeder's websites to get a good feel for how the different morphs look. Here's a couple in no particular order:
http://www.kingsnake.com/boamorphs/
http://www.basicallyboas.com/morphs.html
http://www.classreptilia.com/boa_morphs.htm
http://www.redtailboas.com/morphs/morphs.html
http://www.constrictorsnw.com/boamorphs.html
Hope that helps!
-April
-----
'There is a very fine line between "hobby" and "mental illness."' -Rainshadow