There is also the old classic "Designing Arcade Computer Game Graphics" (
http://www.blendernation.com/2007/01/12/free-book-designing-arcade-computer-game-graphics/ ) by Ari Feldman (published for free in 2001).
It is very technical read though and probably not focusing so much on what "we" think when we think "pixel art book". All the technical information is still relevant(on old systems at least or when trying to emulate the look and feel of old systems which is often what pixel art aims to do these days), especially when the produced art is meant to be used beyond just being a work that stands for itself (e.g. as an asset ready to be in a game).
A "pixel art book" would probably contain step-by-step tutorials about individual pixel art techniques like choosing colors, aa, abstracting shapes, where and how to lay down the individual pixels, dithering/texturing and whatnot and probably also teach some art fundamentals which are relevant for all visual art.
That old book however does not "teach" these things.
Instead it provides a broad range of more high-level knowledge but in a very detailed way which makes it appear low-level again but it is a technical-level, not an art-level focus (except for the color theory and animation parts). Not sure how to describe this better. Read/skim the book and see for yourselves and it might become clearer what I am trying to express here.
The book does not teach steps or skills, it merely provides knowledge but applying the knowledge is mostly left as an exercise for the reader (that is not a bad thing imo for one can not really learn how to do things without trying and seeing for themselves and making their own mistakes instead of repeating someone else's by following a step by step guide).
I think what the book was/is trying to do is close the gap of knowledge between coders and artists but in a way that assumes the reader is primarily an artist and not a coder. It is a very boring read for coders who already know most of this stuff and might be a very dry read for artists who are not that interested in the technical side of things and I dare say it is an impossible read for absolute beginners who have no working knowledge of visual crafting at all.
