OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
The Yemen in early Islam, 9-233/630-847 : a political history by ʿAbd al-Muhsin Madʿaj M. al-Madʿaj. ISBN 0863721028. Published by Ithaca in 1988. Publication and catalogue information, links to buy online and reader comments.