My favourite book in the past was always Easy Game by Andrew Seidman, but I imagine it's a bit dated. Same goes for Doyle Brunson's Super System, which was groundbreaking in it's day, but old hat now,
I read the Jared Tendler The Mental Game of Poker, and enjoyed it, but I'm not a tilty player, so it was of limited use. Maybe I should read it again.
Other authors that I have found useful are Ed Miller, Jonathan Little an, as mentioned earlier, Alex Fitzgerald.