Doing a quick Google Search (bookmaster +xml) turns up a Yahoo Groups thread from 2002 http://tech.groups.yahoo.com/group/xml-doc/message/2752?var=1. In addition, there are several for-hire firms that will do the conversion for you, depending on your time-frame and budget.
in reply to Looking for parser of "bookmaster" script files
As I recall from playing around with Script (SGML's predecessor) conversions, the language was simple to parse -- look for the colon in position 1, extract the next three characters, do a table look up to get the appropriate subroutine (all written in Basic Assembler, mind you) and go do it. Where we got ourselves really knotted up is with 'user extensions', macros that looked like markup, but weren't (:xpl in your example). We finally ended up running the Script Processor with the command-option that expanded all macros inline and then we converted the resultant. It really bulked up some of the documents -- 500 lines going in, 5000 lines of output was not unusual.
I Go Back to Sleep, Now.