This is a Windows Installer document from Microsoft which is available online at their MSDN site and also comes with the "Windows Installer SDK" (although its no longer available as a ".CHM" document).
This documentation is about as good as Microsoft's doco gets (ie not very good) but is still very useful.
It is probably the best source for MSI internal information and for specific information about a table or field. It is also very useful for looking up Windows Installer error codes.
The information is typically badly laid out, partial and typically supplied without context (so basically you ALWAYS have to do more reading BETWEEN the lines than reading the actual lines!).
The latest Windows Installer SDK (for example Feb 2003) uses the MS Document Explorer. To start this documentation use "StartProgramsMicrosoft Platform SDK February 2003Platform SDK Documentation" (then Setup & System AdministrationSetupWindows Installer).
It is best to supplement this with information from other sources such as the newsgroups:
And the Frequently Asked Questions: