This book attempts to provide a pocket-sized reference guide to the Oracle SQL*Plus utility. Within its scope it generally succeeds, being (as far as I could tell) accurate, succinct and clearly written. (Compared with the online documentation, I found the explanation of commands more clear, while being slightly more concise). Another feature of the book that I liked was that it not only gave warnings of potentially dangerous features, but also gave reasons for the danger. I have often found this missing in other reference guides.
As far as this goes, this is a useful reference guide. My reservation about this book is whether it provides value for money, given that I could not imagine requiring this book where I do not have access to the on- line Oracle reference material. Also, I would imagine that any time that I wanted to use this book I would be quite likely to want to refer to (for example) an SQL reference manual, which is outside the scope of this book.
Although the book is better written (in my opinion) than the standard documentation set, whether this justifies the cost will depend on the prospective reader.