This book describes the OPEN (Object-oriented Process, Environment and Notation) modelling language or notation.
OML is a visual modelling language and all the elements of design diagrams are described at length. However, the book is exactly what its title says 'a reference manual'. If you want to know exactly what an OML diagram is telling you, then this is the book for you. If, however, you want a tutorial on OML notation then this book is not suitable.
The appendices discuss the difference between OML and UML, which should be of interest, but the material is still aimed at the notation lawyer and is unlikely to be of great interest to the general reader.
I think that the book would have been much improved by a largish project described in OML, to introduce the notation.
In conclusion, if you need the precise syntax/semantics of OML, then this is the book for you. If you just want a tutorial guide to OML notation then avoid this book.