The AUCTeX manual written in Texinfo is included in the distribution. Follow the link to read the online version for the current stable release. The manual is available in various formats, e.g. HTML, PostScript or PDF, if you don't like the info manual accessible with C-h i d m AUCTeX RET in Emacs.

Frequently asked questions are answered in the FAQ section.

The preview-latex manual is available online as well. You can access it from inside Emacs by typing C-h i d m preview-latex RET.