[kmike84]

In modern PSD files a lot of information is stored in "descriptors"; they are all parsed in an uniform way. Some of the information (like font data) is in undocumented "Raw Data" descriptors though.

The official PSD specs are now online: http://www.adobe.com/devnet-apps/photoshop/fileformatashtml/.

Specs don't provide information about all PSD features, but they are a good start. As I recall, an initial version of https://github.com/kmike/psd-tools took a weekend; I was following specs, using PIL (Pillow) for pixels handling; also, I was checking other PSD readers when something was unclear - there is a lot of open-source PSD readers. Earlier readers are from 2005 or maybe earlier, they have been available long before the code with this "famous" comment was written.