AMB stands for "Ancient Machine's Book". It is an extremely lightweight file format meant to store any kind of hypertext documentation that may be comfortably viewed even on the most ancient PCs: technical manuals, books, etc. Think of it as a retro equivalent of a *.CHM help file.
This web page holds the format specification, as well as reference tools to work with the format: AMB (the reader) and AMBPACK (archive packer/unpacker). All tools are published under the terms of the MIT license.