The 'Companion' covers the first four books in the series. There is also a background from Diana Gabaldon on how she came to write the books, which is interesting. Here you can read about the Synopses, the Characters, the Family Trees, a guide to the glossary, web-sites and online venues connected to the series, Research, where the titles come from, the view from Lallybroch, FAQs and more. It includes 'The Gabaldon Theory of Time Travel' which should be interesting.
I would like to go into this book right away, but alas, have some other books that I absolutely have to read. This book will be my breakfast reading. Hmm, but it might mean that my breakfast will be rather long!