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.
TIFF Technical Notes can be retrieved from: