The tbt filetype is basically a bunch of strings mashed together with a leading pointer table. There are only four known examples of it on the Cross ~CDs, and the exact details of the format differ from file to file.
tbt files have no true header—the pointer forest begins immediately. Each pointer indicates the beginning of a string, relative to the beginning of the file. There is an EOF pointer. Pointers are little-endian (surprise, surprise) and usually 2 bytes, although 0018.001 uses 4-byte pointers.
The strings that form the meat of the tbt file may be relative alphabet encoded or plain ASCII, depending on the exact file. They are always null-terminated. Strings may contain line-feeds or other control characters.