On the positive side: if you are an OpenERP user or developer, you will probably want this book as a reference on your bookshelf, because even though most of the material is available on the web anyway, you still can't beat paper, in my opinion.
On the negative side, this book is quite a dry read, and is very light on explanation of what the software actually does; it does tell you what to do for straight-forward use of the software, but does not cover all options.
E.g. on page 147, there is a screen shot for receiving a supplier's order, and on that screen you can see buttons labelled "Process Later", "Process Now", "Unreceived Products", but there is no explanation of what "Unreceived Products" actually does - does it show you a list of products to add to the form, or what? It does not explain what the Reference field used for - is it just for your reference, or does the system allow searching on this field? What are the implications of each of the options in the "Invoice Control" field?
So the book is very shallow and incomplete in its explanation of OpenERP.
The book was obviously written by a non-native English speaker - there are a lot of unusual idioms, e.g. on page 146 it says "You'll find a list of all the awaited receipts..."; normal English usage would phrase this as "You'll find a list of all outstanding orders...". There are many instances of weird English.
Other than that, it's a useful reference for OpenERP. It was written for version 5 of OpenERP, and the material is still relevant for the version 6 of OpenERP, even though some of the functionality has changed and the screens have been refined.
My verdict: worth buying if you're a user or developer of OpenERP.