The following is a set of links to sites and documents with useful information about Reflectance Transformation Imaging (RTI), including sites that refer to it by its original name, Polynomial Texture Mapping (PTM).
Cultural Heritage Imaging – This should be your first stop. CHI is a San Francisco-based non-profit that has spent much of the past 15 years developing RTI techniques and software, and running training sessions. Go to this website for general info, and for software/manuals for processing and viewing RTI data. All info and software is free, and freely distributable. The forum at the site is a great spot to get answers to questions.
Google – Searching for reflectance transformation imaging or polynomial texture mapping will bring up hundreds of useful links. I also recommended checking out the “image search” tab, as you’ll find lots of cool pictures of artifacts/objects imaged using RTI. “Scholarly articles” near the top will bring up links to academic papers.
Building your own system – I’ve just started a project page on Hackaday, which will ultimately have full instructions on how to build your own inexpensive RTI system.
HP Labs PTM Site – The technique was developed at HP Labs in 2001 by Tom Malzbender and Dan Gelb, and while I believe both have left HP since then, the original info page is still up. Includes links to downloadable RTI files of parts of the Antikythera Mechanism, which RTI played a key role in deciphering. There’s a PBS Nova special on the Antikythera Mechanism viewable here; fast forward to roughly the 26-minute mark for the section on RTI. There’s also a link to the original paper introducing the technique.
While the CHI RTI viewer is the best currently available, the Inscriptifact project also has a freely-available viewer with a few additional options not available in the CHI viewer (e.g. two individually-configurable light sources). Requires Java.
Multi-Light Imaging For Heritage Applications – Nice PDF manual describing the technique, with use examples.
Archaeological applications of polynomial texture mapping: analysis, conservation and representation – Paper from the University of Southampton talking about applications of PTM/RTI; one short section on lithics.