I've been warned by a Sumerologist that the book is idiosyncratic. It does seem to be the only in-print and inexpensive option aimed at amateur scholars.

More mainstream options:

Hayes, John. A Manual of Sumerian Grammar and Texts. A teaching grammar for the beginner. In print, but quite expensive.

Edzard, D.O. Sumerian Grammar. A reference grammar rather than a teaching grammar with exercises and the like. It's really good if you already have some familiarity with linguistics. It's in print and inexpensive.

Thomsen, Marie-Louise. The Sumerian Language: An Introduction to Its History and Grammatical Structure. A reference grammar. Out of print but available on the high seas.