http://www.logos.com/product/33588/ethiopic-grammar-2nd-ed-enlarged-and-improved
Dillmann's Ethiopic Grammar is in community pricing at the moment.
I am a beginner at Classical Ethiopic, but I have gotten the picture that while this grammar is old it is still essential for any student of Ge'ez. Classical Ethiopic is a Semitic language. Don't let the script intimidate you, the language isn't any harder than Hebrew. It may help you at Biblical Hebrew too, you can compare the languages, and eventually read the Bible in Ge'ez.
One might at some point want a more recent grammar, like Lambdin, but Dillmann uses the Ethiopic script from the start, so you get familiar with it right away.
I don't think that the A or B companies still have anything Ethiopic, so having this in Logos, would certainly be nice.