This is probably one of my favorite books and, in addition to the Oromo Newspaper Reader and Lexicon, one of the first books about afaan Oromoo that I ever read focusing on the grammar aspect of the language.
Grammatical Sketch is clearly a reference work and it would be unlikely for someone to read through the entire book from cover to cover. It would not be able to be used as the sole book for learning afaan Oromoo but it would be one of the books that I most highly recommend. You can look up specific points to better understand why something is phrased that way. If you do want to attempt to read the entire thing you could then look for examples of this within the books you are reading.
The book does bring up some interesting points I would like to go through in future linguistics posts. It is only mentioned here and it seems to be relatively ignored in many works but the idea of root words is helpful from a language learning perspective. Admittedly roots sometimes fail but to look at a root such as bar– and be able to link it to barachuu, barsiisuu, barsiisa, barumsa is a helpful skill.
Griefenow-Mewis also has a textbook for learning afaan Oromoo, and basically the only one targeted towards adults that has been published with the most common qubee spellings, but it is in German. My German is not great but one week we will also go through that book.