Ok, I have not read the official IBM Cognos TM1 documentation, so I cannot make a first hand comparison between this book and that. Other reviewers have suggested that the book offers little different.
Garg's offering might seem to naively and initially portray TM1 to be in part a souped up spreadsheet. Several sections show a typical spreadsheet layout, with a table populated with data. But TM1 goes far beyond being a pure spreadsheet. Crucially, it has scripts that run in the backend to hook up with the TM1 Data Server. The latter contains what IBM simply calls 'cubes'. An inspired choice of terminology. Compact, with a simple metaphoric visualisation. A cube is considered to be business data in some type of multi-dimensional format. This is not the lowest form of data. The cube is derived from underlying data sets residing in, for example, a CRM source database. The Data Server goes out to this and constructs a cube.
The various user interfaces that the book spends much/most of its time on are the views that TM1 lets you have for the cubes. Kudos to IBM for eschewing complex jargon. If I do seem to be emphasising this, it is because notation and terminology can and probably will affect how you use a software package. The simpler and more self evident the jargon, the easier it is for you to mentally build upon it and to leverage off the user interface.
The user interface shows how to make a cube. Quite different from a traditional relational database approach of making interrelated tables and using standard normal forms. Arguably, IBM's approach with the cube may be more intuitive for the business user unversed in database theory. In and of itself, you might want to carefully peruse this book, to see if this approach is easier for you. Where possibly you can apply your business expert knowledge in an easier way.
This is something I think the book does not emphasise enough. If you read this book, at some point, step back from what are low level details about a given set of steps in the user interface and consider the bigger significance of what TM1 lets you do. That clever avoidance of explicit relational ideas can really expand the audience and potential usefulness of TM1.