As the only mass-market book on the T4 I could find, there's little competition, but that is no excuse for the poor standard of this book. Although the major compnents are covered and essential information is provided, it's disappointingly superficial.
The electrics section is particularly poor with badly-reproduced, fuzzy diagrams of only a selection of circuits (e,g. the MFI is covered but the power windows and central locking are not).
The book says that it is intended to supplement the regular owner's manual, but only an owner with extensive mechanical experience would be able to work with the limited information given.
While a book as short as this will naturally be limited in scope, this is no excuse for the many typos and simple errors in the text. The omission of the word "not" in several places where it is required is particularly worrying.
In short, this manual is better than nothing, but only just; buy it if desperate.