This book provides students with an accessible introduction to Hallidayan functional grammar and prepares them to use Halliday's Introduction to Functional Grammar (IFG) later on in their studies. In this updated edition, Thompson focuses on changes in two main corpus data and system networks.
This entire book was designed to explain Halliday and Mathiassen's Functional Linguistics tome. Which is kind of absurd, but definitely needed. Thompson's writing is much more accessible, but his book is 1/4 the size of Halliday's so it is in no way inclusive of all elements of functional grammar. I would recommend this as a needed companion text if you are planning on attempting Halliday and Mathiassen.