No one has yet directed your attention to a superb resource on this very site:
ftopic11429.phpClick on the very first link in the first post.
Your question is quite a common one, and the answers you've had so far reveal a bit of a problem. There actually isn't a single resource that really helps people who are just starting out.
Practical Mental Effects and
13 Steps to mentalism are both treasure troves of information. Personally, I don't find them to be outdated (the language used doesn't bother me), but as naychandler1 says, that sort of work can be overwhelming.
The problem with many other works (books and DVDs) is that they often teach you "some tricks" (for example, Andy Nyman's excellent
Get Nyman). You can come out with some great things to perform, but little real grounding. You might pick up stuff along the way, but it's not a particularly efficient way of developing your background knowledge. Other books and DVDs focus on one or two aspects of mentalism (for example,
Multiplicity, or Elliot Bresler's amazing work on billets,
Switchcraft). If you look at Craig's List (no, not that one, the one I referred to above), he refers you to a lot of resources that cover particular techniques in detail.
I suggest not going mad and buying loads of stuff at first. You've already said you are getting
13 Steps and I think that's a good place to start, because it gives you information on many (but not all...) of mentalism's classic techniques. For the price, you cannot get a better start. There are a couple of things I will point out about it. First, it was originally designed as a course, so each chapter was to be provided and read individually. In other words, take your time over a chapter. Second, you don't have to read every chapter and you certainly don't have to do them in order.
You may find your magic background stands you in good stead. Dr Todd Landman, co-founder of the BSME/Psycrets and contributor to this site, is a great believer that magical techniques and knowhow greatly enhance a mentalist's skill set. It may be that you will benefit from starting with areas that make use of abilities you already possess.
One other thing - you may need to think very carefully about your presentation, especially if you are switching to mentalism from "conventional" magic. It could be that you will really benefit from reading a couple of books about performance style/scripting more than you would from works dealing with techniques or tricks. These days, I tend to be as interested in hearing about a performer's approach to choosing material and presenting it, as I am about new tricks.
Actually, that's a lie. I am
more interested in it.
In particular, think carefully about whether you want to mix mentalism into your magic sets. The conventional wisdom goes that you can undermine the impact of the magic (because, once you've read someone's mind, they aren't too interested in where that lemon came from), but you also weaken the mentalism because you're implying that it is trickery. It is probably not as clear-cut as that, but it's something to think about.