In my opinion, and it is only that, this book is effectively the second half to the main rule book. It expands on a lot of the basics in the main book. It is probably the only book other than the main rule book I would mark as a must have.
I do write a lot of my own rules and change things as I see fit anyway, but this book has eliminated a lot of that need. Expanded skills, expanded items, expanded homeworlds and expanded character options.
If you buy any one book to compliment the main rules, buy this one.