You'll learn faster from teachers than from books. Join us on IRC. #LP101 on slashnet.org If you really want something to read, dig through the "Pick Fu" and "Questions" sections of this forum, as that's where most of the "So I found that if I did
this, then I could open
this" or "I'm stuck on
this, is there a technique to overcome it." It's funny, I'm setting up a lock library at a hacker space right now, and they keep asking me what books they should get...and there really aren't any I would recommend, save for Graham's High Security locks book & Ollie's Impressioning book. Even then, neither really teach how to pick locks, one is to broaden your knowledge of lock engineering & the other to teach a very specific opening technique, not directly related to picking.
I actually wouldn't recommend LSS+ to anyone to learn lockpicking, even high security lockpicking. It's a tome, with a lot of superfluous language. It's interesting, to be sure, and epic in scope, but really, after my initial fascination with it when I picked it up at a con a couple years ago, I haven't referenced it once.
Learning how high security locks function, and then reading about how various attacks have functioned over the years, is probably the best course of action for your Primus goals. Two great resources in that regard:
http://blackbag.nl/ - Barry Wels' blog. He's the head of the Open Organisation Of Lockpickers and always at the bleeding edge of exploit research.
http://toool.nl/Articles_by_Han_Fey - Han Fey, one of the preeminent lock collectors in the world, and the #2 guy at TOOOL, writes brilliant white papers on high security locking systems. He hasn't covered Primus specifically, but his papers do a better job of getting your head around how high security locks are conceived, built & iterated than LSS does. Plus, they're free!

So, yeah, Pick Fu, Questions, join us on IRC, Barry's Blog & Han's papers. And it's always worth checking out the local section to see if anyone is picking near you, or seeing if any of the clubs have chapters in your area.
For the Primus, specifically, you can find really nice breakdowns in the "Locks" section of this forum, and if you keep to the above resources, and contribute on here, it won't be very long before you have access to our advanced forums and see how various members have attempted to attack it. There you can throw out your ideas, get feedback, etc.
Good picking, and you're always welcome to drop me a PM with any questions, too.