This site may provide you some helpful informations (at least which I found helpful) in the beginning.
Since I do not know any good CAPL books (but honestly didnt searched the net very well), I think we have to go with the stuff Vector and the community is providing us.
(Vector, 216 pages) This is the basic documentation from Vector about the CAPL programming language.
CAPL_Function_Reference_Manual
(Vector, 179 pages) This is a function reference manual including thre description of important CAPL functions.
Also, it might be a good idea to check the other documents Vector is providing to us and see, if something is helpful there. To do so, check
https://vector.com/vi_downloadcenter_en.html
(for Technical Articles, Product Descriptions, Press Releases, Application Notes, Support Notes …). Since I found some of the „Application“ and „Support Notes“ very helpful, you can just search for them in Google:
vector an-ind filetype:pdf (Application Note)
vector sn-ind filetype:pdf (Support Note)
robertscaplblog 