My favorite butler book is not a part of the genre, but rather a biographical account of a real butler who lived in America and served James Madison.

http://www.amazon.com/Slave-White-House-Jennings-Madisons/dp...

Another excellent historical account of the servant class, this time in England, is this gem:

http://www.amazon.com/Servants-Downstairs-History-Britain-Ni...

I have not actually read any of the butler novels that I mentioned but the one I hear mentioned every time this topic comes up is The Inimitable Jeeves:

http://www.amazon.com/Inimitable-Jeeves-Wooster-Book-ebook/d...

Articles crop up from time to time about modern butlery, which is enjoying a renaissance as Chinese billionaires search for ever more gaudy ways to spend their vast fortunes, like this recent GQ piece:

http://www.gq.com/story/rich-billionaires-butlers-servants