Usulaa refers to the roots, the methodology, or the scientific rules governing the language. Think of it as the equivalent of The Chicago Manual of Style meets Oxford English Grammar , but specifically for Oromo.
(May it be useful to you.)
The Internet Archive is a digital library. Search for "Kitaaba Usulaa." Sometimes, out-of-print copies (pre-1991) have been scanned by users. These are legally gray, but if the book is completely out of circulation, it is often available for "borrow" digitally. Kitaaba Usulaa Afaan Oromoo Pdf Download - Google