A copy of the 6.0 specification is available from Adobe at http://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf, or from the libtiff ftp site at ftp://download.osgeo.org/libtiff/doc/TIFF6.pdf.
Draft TIFF Technical Note #2 covers problems with the TIFF 6.0 design for embedding JPEG-compressed data in TIFF, and describes an alternative.
Other Adobe information on TIFF can be retrieved from: http://partners.adobe.com/public/developer/tiff/index.html
Joris Van Damme maintains a list of known tags and their descriptions and definitions. It is available online at http://www.awaresystems.be/imaging/tiff/tifftags.html
There is a FAQ, related both to TIFF format and libtiff library: http://www.awaresystems.be/imaging/tiff/faq.html
There is a preliminary BigTIFF Design for a TIFF variation supporting files larger than 4GB.