Beauty is subjective but you could pick up some books on design principles in general

It's been ages since I read it, but The Dynamics of Architectural Form, by Rudolf Arnheim, is an interesting read.

Seki's and Ramachandran's work in neuroaesthetics is worth checking out. http://en.wikipedia.org/wiki/Neuroesthetics.

And by the way, "beauty" isn't really that subjective. But aesthetic responses do involve values, which are highly individualized.

Beauty: A Very Short Introduction by Roger Scruton.

On Bullshit, by Harry G. Frankfurt.

Read some of Pallasmaa and Zumthor's books. My personal favourites are The Eyes of the Skin, Atmospheres, & Thinking Architecture.

Mark Foster Gage wrote a book called "Aesthetic Theory: some long subtitle that I can't remember." It's got a lot of excerpts from people like Kant and Burke in it. Seems like a decent starting point.

I actually read the books by J. Pallasmaa and P. Zumthor, but I will definitely check out some of the other recommendations. 

Thanks for the recommendations, greatly appreciated!  

I would highly recommend Homemade Esthetics: Observations on Art and Taste by Clement Greenberg - an excellent book.

Wabi Sabi, by Leonard Koren

WABI SABI is a great book. It appears as though Mr. Koren is prepping a second book on that topic.

I also like his ARRANGING THINGS (which isn't necessarily the type of book you're asking about) and UNDESIGNING THE BATH.