well the book isn't out yet, supposedly september 9th unless something goes wrong. And it should be really good, I can't wait.
2.g3 English making a comeback

Indeed, Grandmaster Repertoire 1.d4 is absolutely fantastic and Mikhail Marin is both a great writer and an excellent 1.c4 player.

In my opinion, 1.g3 with equality is the safest and most effective move order to guarantee an "edge in understanding", or to be more in tune with the "spirit" of the positions. You often maintain the flexibility to choose from the closed sicilian complex, english opening complex, Tarrasch QGD's, Reti. etc. Or Black may choose to play for an advantage, which usually results in the player playing ...g6 black systems, even the closed sicilian reversed (basically the English opening however). Instead of spending the opening trying to equalize, you can be thinking of how to improve . The KIA, Benko/Benoni reversed, Reti, etc. Maybe you wouldn't take it to corus with you to win, but by playing it you guarantee yourself chess improvement and comfort in diversified situations. The great thing about a consistent development of a minor piece (Bg2) is that reaching diverse but similar structures expedites your education, as you see what exactly makes a plan or tactical operation, etc, succeed or fail in chess, not just a pet opening. Everyone should try 1.g3 sometimes.
Students will recognize the necessary conditions/variables for the common tricks in the numerous transpositions to other book openings and/or reversed defenses as she or he acquire more "Bg2 experence". The constant and usually active piece placement of Bg2 ensures the student a progressively deeper and more diverse understanding of the "spirit" of the positions. Awareness of both obvious and abstract differences between Black's setups and how best to react is very educational, and I while Nf3 is similar, 1. g3 is most diverse.
It pays to play openings that don't force a response from Black. Some others to choose for the aspiring student of chess:
- Collee System
- London System
- Torre Attack
- Trompowsky Opening
- Stonewall Attack

I always open 1.c4 because I'm trying to learn the ins and outs of the opening. I am also really enjoying the positions that result from it.
I think 2.g3 is becoming my preferred second move, of the choices available in, so I will have to check this book out - thanks for the heads up OP.

I like playing the English and have always prefered 2. g3 after 1...e5 to try and get a reversed Dragon/Closed Sicilian game.

But this book doesn't cover the Symmetrical English, right? The Symmetrical English is the reason I don't play 1. c4.

The symmetrical is covered in "Grandmaster Repertoire 1.c4 vol. 2" which is coming out next year. It covers all responses to c4 except for 1...e5

Actually I think the 2.g3 English opening lines are an excellent choice for the theory-averse player. They promise white more variety of structure and style than the non-mainline 1.d4 openings, and aren't nearly as theoretical as the mainlines.
With Kosten's "The Dynamic English", Nigel Davies' chessbase DVD, and now Marin's Grandmaster repertoire, things are definitely looking up for this way of playing.

I don't suppose someone would care to elaborate about the theoretical significance of 2.g3? I usually play 1...e5 against the English myself, and practically everyone plays either 1.c4 e5 2.Nc3 Nc6 3.g3 or 1.c4 e5 2.g3, with an eventual Nc3. Is there some particular drawback to the latter move order? Is 2...d5 a big theoretical problem? What advantage would/could 2.g3 provide over 2.Nc3 3.g3? Avoidance of a 2.Nc3 b6 reply? Does anyone really play that?
Seems like all the same thing to me. I've never really thought of the English as an opening where move order needs to be precise... to me, it's more one of those sluggish flank openings where White's going to play these 6-7 moves no matter what Black does, and then we'll start playing chess.

I know next to nothing about openings. I choose c4, because I had heard that the sicilian is really popular and lines of theory go 20+ deep. Obviously I wanted to avoid this and picked 1.c4. I mostly get met with 1...e5 and have nearly always replied 2.g3 and only now do I hear that that's a totally rubbish opening. Although, you say this book promotes the 2.g3. Maybe, I am onto a winner after all :)

I mostly get met with 1...e5 and have nearly always replied 2.g3 and only now do I hear that that's a totally rubbish opening.
It's not a totally rubbish opening. In fact it's very solid and gives great defensive structure with open counter-attacking play. 2.g3 allows the bishop to be strongly and quickly developed on move three, whilst at the same time preparing the defensive cordon (great tempo move!), and the king to be castled on move 5 (depending on replies). Plus, as has been said, this opening is fairly fluid and transposition is possible.
I play this all the time as white and I love the positions that develop, which always seem different to me - maybe I haven't seen enough games yet. It certainly seems to generate more unique middle game positions from the same opening lines than some other, more popular openings.
All in all it's an opening that (most) opponents wont expect which gives you the advantage of novelty plus all the above - not rubbish but great in my opinion!
For those of you who play or are interested in playing the ever-popular 2.g3 systems in the English you might want to have a look at the new "Grandmaster Repertoire 1.c4" by Mikhail Marin. The book just came out and I was very much surprised to find that he advocates the 2.g3 move order against 1...e5! Of course just as I change my white opening repertoire away from the English a book comes out that fixes all my theoretical problems!
Anyway, those of you who play Tony Kosten's "Dynamic English" repertoire will definitely want to check this high quality book out. And those of you who thought the 2.g3 lines were too kiddy (like me!) may also want to check it out.
http://www.qualitychess.co.uk/products/6/48/grandmaster_repertoire_3__the_english_opening_volume_one_by_mihail_marin/
http://www.qualitychess.co.uk/ebooks/GMRep3-Englishvolone.pdf