Sorry I didn't see this earlier Nino.
Well, there are always different opinions on this. I'll add my own of course, but before I do here are two others:
http://www.lybrary.com/thirtynine-steps-mentalism-a-10.html
http://gunnsight.blogspot.com/2004/08/gunns-beginners-guide-to-mentalism.html
I agree with certain parts of both guides personally, and disagree on some points too. Bob Cassidy's focus on the classics is well placed, and has chosen a very good selection of foundations of mentalism. As mentioned in the article, some modern ones are left out, so I'll leave it to you whether some "modern classics" should be included or not. Gunn's article makes the excellent point that the best mentalists have a good working knowledge base on which they can develop as well.
So, as for what I recommend for the very start... I mostly agree with Bob Cassidy on this point, but there's the obvious fact that it lacks his own works... Therefore, I have to agree with D ICE R in saying, and consistently saying for a while now, along with others, that the single best starting point to mentalism is Bob Cassidy's Fundamentals.
Why?
Fundamentals doesn't teach you mentalism so much as how to be a mentalist. It's brilliant because it teaches you
about mentalism - what it is, what it isn't, and
how to perform it - because mentalism is very different from normal magic, and succeeding in the two fields takes different skills. Mentalism is unique in magic as it is arguably the last remaining field of magic which can be considered "real" by objective, logical audiences. Then there is the fact that you cannot get more personal than reading someone's mind. Because of that, it is fundamentally (no pun intended) different to magic in terms of how it should be handled, and performed, for the best impact.
The reality is that people like Derren Brown, to name one famous example I hope you can relate to, take tricks that read boringly from books very few people read (Annemann's Jinx, for example) and fools the pants off 99% of magicians/mentalists and 100% of laymen. Fundamentals gives you the tools to do the same.
Mentalism for example places a much greater emphasis on persona. Another more specific example is shuffling. Many (but not all) mentalists shy away from things like faro shuffles, even tabled riffled shuffles, etc. Why? Because they realise that cards are associated with two things, gambling first, and magic second. Mentalism is meant to take place in your mind. Any allusion to some sort of dexterity or skill with the cards degrades it to the level of a card trick - and instantly, perhaps the single biggest strength of mentalism, the reality, is gone. There are a lot of keys to performing strong mentalism - and making it "real", in the sense that it should rise above being tricks. Fundamentals teaches you about mentalism, because without this understanding and internalising this way of thinking, it is impossible to get the most out of it - you will instead be performing mere tricks.