Why wait? It's gonna make certain encounters and dialogue so much more meaningful if you go ahead and read.
It's just in case I find I absolutely hate a character, but get in game and find they are presented in a totally different light, but not be able to get past my first impression. From what I've heard, many ME characters were presented in very different lights in the different media than in the games, and I think there's a chance this may also happen in Inquisition. So basically, at this point, I think my first Inquisitor will want to support Celene, but if I read the book and loathe her, I would change my character with metaknowledge and stuff and I don't want that at all.
I've done it in the Walking Dead game, I'll avoid spoilers, but there was a character that I already knew and loved really and it affected how my Clementine interacted with them. And even though, she shouldn't trust them, I do so she did and will continue will, and I am determined to keep them alive at all costs because of something I know, but shouldn't.