A portable free ANSI C JBIG lossless image compression library and JBIG<->PBM conversion tools known as jbigkitis available.
JBig hardware, a chipset, from
PIXEL Magic 138 River Road Andover, MA 01810 508-688-4410
Other Link Pages
Vital statistics: collected information about expected compression ratios for various combinations of data and compressor can be found at the ratios page, and also as the results of the archive compression test
If you want it (almost) all in one convenient reference, I recommend Encyclopedia of Graphics File Formats/Book and Cd Rom. ~Murray, James D./Van Ryper, William~. 07/94, PAP. $59.95. A fat text, at a fat price; and a CD Rom full of source code. All stuff you could find on the internet, but it's here in one place.