I expect a quite small market, but it is (as far as I can tell) the only book on reading and understanding specifically Arabic coins. It's a book originally published in 1973, then revised in 1980, by Richard Plant and it's been in and out of print a couple of times since then. It was originally also translated into Russian! I want to provide affordable assistance to inquisitive people both now and for future generations, so hope to keep it in print in one shape or another for eternity, even if I won't be able to retire from it! For a few quid it can become part of your field or experience.