I know I say this a lot about certain topics, but a deep dive into the particulars of IPv6 could fill its own book, so I have to pick and choose for the discussion here. That said, I will cover some introductory knowledge that will be useful for further research. As always, my advice for IPv6 is to read the authoritative RFCs. RFC 2460 was the original detailed definition and description of the new version, but it was a Draft Standard for all those years. The levels of Standard refer to maturity of the technology being defined, with the Proposed Standard being the least mature, and the Internet Standard being the gold well, standard. IPv6, after those long years, has become an Internet Standard with RFC 8200 (STD 86) as of July 2017. Though I certainly encourage reading RFC 2460, it is now officially obsolete.  

IPv6 is important to the pen tester for two big reasons: one (and hopefully most obviously), it's the newest version of the internet, so you're only going to see more of it; and two, as with many newer things that haven't quite replaced the predecessor yet, it's not given the same level of security scrutiny in most environments. Many administrators aren't even aware that it's enabled. You might get some useful findings with just basic poking around, and regardless, you'll help raise awareness of this new protocol.