There aren't that many books written about hockey umpiring but Tim Craffer has written a series on various aspects of umpiring and coaching of umpires. They are well written and easy to read, and represent excellent value for money.
Ordering details from the author at :
Phone: 01328 830 242
A book about soccer refereeing may not at first sight have much of interest to the Hockey umpire, but this book by the World's best known referee makes interesting reading and gives an insight to the psychology of refereeing (and umpiring) and the mental approach taken to officiate top level games.
Well worth £12.99 from all good bookshops