I should add that both of those above are older books. I know that showing hasn't changed much since they were written, but there might be the odd thing that is now out of date so do be aware of that.
Trimming whiskers is maybe an area where things have moved on. Do check the rules for the class where you would compete, I know that it has been banned by the AHS for their shows for several years for example.
Another vote for 'Showing Horses and Ponies' by Robert Oliver.
Katie Jerram's book 'Showing' is also a great point of reference.
Lastly, I love the Allen Photographic Guides, they have done one for just about everything. No one book goes into too much detail but if you're looking for an overview, these are fab.