A copy of the 6.0 specification is available from Adobe at http://partners.adobe.com/asn/developer/PDFS/TN/TIFF6.pdf, or from the libtiff ftp site at ftp://ftp.remotesensing.org/pub/libtiff/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/asn/tech/tiff/index.jsp
There is a FAQ, related both to TIFF format and libtiff library: http://www.awaresystems.be/imaging/tiff/faq.html