Every published book serves a specific purpose - Inside SQL Server's purpose is to illuminate the internal architecture of SQL Server so that developers and admins can make better educated decisions and develop better databases. Within that purpose her book is excellent and well written.

Is it all things to all people? no. Does it do an excellent job of meeting its purpose? without question, yes. There are some reader reviews that seem to criticize Kalen's work for not meeting other needs, and I don't understand these comments. If you need a general SQL, T-SQL programming tutorial, XML/SQL, or introductory SQL Server book, Inside SQL Server is probably too focused on the internal workings of SQL Server and I'd recommend that you begin with one of the other excellent books on SQL Server. But, if you want to get deeper into the internals so you understand why you're writing code or tuning indexes, then there can be no question that Kalan's book is a must have on your shelf. Sooner or later, if you're serious about SQL Server, you should read this book.