The first thing people learn about Tamil is the fact that this language has its own script and that the script has 247 letters. And that is the point when a lot of people already give up saying that the language is too difficult to learn.
In reality, Tamil is not really difficult. Yes, the script has 247, but they follow a systematic order so that you will be able to read and write almost all of them once you learned the first 30.
Also, Tamil is a really old language, it is one of the oldest still spoken languages. That makes learning this language easier than modern languages. You know why? Because a lot of old languages had the goal to standardize the way people communicated, therefore the language followed strict rules. You will find out that Tamil as a language has a lot of simple rules. Once you understand the rules you will be able to understand almost all of the language since it does not have that many exceptions.
The longer a language lasts the more it changes. Driven by the circumstances of the people that speak the language some rules are modified, exceptions are added or rules are even dropped entirely. That can happen from place to place, dialects emerge.
The same happened and still happens to Tamil as well. The South Indian Tamil has changed a lot from the original language while in Sri Lanka the changes happen way slower. But the changes happen only on a spoken level.
The book teaches you the grammar pure Tamil that will help you to read, write and understand Tamil not matter where they were written. Using the basics you learn in the book will help you to learn to speak the language.