For some of that, there's the Complete Guide to the Fey (Goodman Games) and Faeries (Bastion Press) - both out of print, but occasionally still available (or constantly, as PDFs.) Worth a look.
However, if you want to keep things closer to the standard D&D way of approaching fey, you might want to steer clear of both of them.
Into the Green (Bastion Press) - also OOP - deals with many things forest-y, but I'm not as familiar with it, as I only have Into the Blue, from that range.
Mongoose Publishing put out a series of books, the Slayer's Guides - again, OOP, I think - and I believe one of them covered Centaurs.
Hope that is a useful start, at least.