What you want to select is a link to be able to jump to the next letter. And selecting links was only possible to do with two buttons which was a bit disappointing since I had hoped that a grid selection scheme would work.
I you only want to select one letter a grid will work.
If anybody want to test my four letter selection document it is available at:
http://www.ida.liu.se/~tompe/ebook/search.mobi