Wilber is a self-proclaimed pandit (rather than a preacher) on pretty much all things spiritual, psychological, sociological, and philosophical. He's made extensive maps that tie everything together. In terms of personal beliefs, he is pretty close to Plotinus---IIRC, he's said as much. Plotinus was one (if not the one) of the neoplatonists, basically taking Plato's ideas of the world of forms (round, red, etc.) one step further and claiming the forms all derived from "The One". That's a kind of monism that underpins much of medieval Christian thinking. A tiny difference between Plotinus and the nondualists is that Plotinus saw The One as something that could be approached with philosophy, that is, the better the philosopher, the closer they are to The One. Whereas non-dualists believe that they can essentially merge with The One because they are not separate from The One.
The Sex, Ecology, Spirituality book is often mentioned around here, but I think that's a pretty heavy starter book. Instead I'd recommend his "No boundaries" book. Perhaps also "One Taste" as a light read.