Is "Introduction to Elementary Particles (2nd Edition, 2008)" by David Griffiths still relevant to start with particle physics? The book is generally recommended, but it's from 2008. Are there any big discoveries or changes in the field that might make the book wrong at some points, or incomplete to a level that it's not useful to study the material?
I currently have the book, and the question essentially is: should I study this book, or go buy a more recent one?
 A: Particle physics is a broad term, which can be used to describe, for example, more phenomenological research to say, research beyond the Standard Model like supersymmetric field theory.
Regardless, a strong foundation in quantum field theory is necessary, and while Peskin and Schroeder is a classic, I would recommend Quantum Field Theory and the Standard Model, having read both.
The latter I think is ideally organised, and the topics introduced in the right order. It will give you a solid grasp of the Standard Model, with the necessary background in quantum field theory. It also gives a good account of modern tools like effective field theory and the background field method.
If you are interested in the forefront of research in scattering amplitudes, it also provides an introduction to the spinor-helicity formalism, which is sort of the beginning of the story of modern methods in this area.
Once you have a solid grasp of quantum field theory from the book, you'll be able to branch out into the areas that interest you most, and read more specialised textbooks without going over the background again.

I will add if particle physics is of serious interest, even if you do not plan to look into more exotic research beyond the Standard Model like higher-dimensional theories and supersymmetric theories, a grasp of representation theory is absolutely vital; I recommend Fuchs and Schweigert.
The whole book is necessary if you're planning on theoretical physics, as opposed to say phenomenological particle physics, but if it's the latter, then you only need chapters 1 to 6, chapter 9, and chapters 15 and 16.
A: The 2nd edition of "Introduction to Elementary Particles" by Griffiths is in my opinion still an excellent book. Quite surprisingly perhaps, even the first edition of the same book (1987 if I recall correctly) is still relevant, except for one or two outdated chapters in the middle about quarks. But these have been updated in the second edition.
Of course when one says a book is good or not one must also ask "what for?". I would say Griffiths is an excellent introduction to particle physics for those who already have a good background in quantum mechanics and some relativity theory, and they are approaching quantum field theory. The latter subject is quite complicated and very technical, and it's easy to lose track of why we're doing certain things, what quantities we can measure, and more generally about the big picture of the basic conceptual ideas in particle physics. This is where Griffiths comes in: it explains the basics of cross sections, decay processes, particle families and so on without going into quantum field theory.
As for what the book isn't, it isn't a book on quantum field theory, and as such it can never be more than an introduction. At some point, if you really want to understand particle physics at a theoretical level, you'll have to learn some QFT. It also isn't a book on the experimental side of particle physics, for that you'll need a different book. 
