They're not sidekicks! They're not assistants! They're not partners! They're not associates!...
THEY ARE COMPANIONS!!!
After fifty years of Doctor Who presenting stories in all kind of formats: TV, Novels, Comics, Audio Dramas, etc... while the term hardly has been used on screen (or any other media) and it's more of something managed by the production team, promo material and news. It's common knowledge by fans of the franchise that any person who travels in the TARDIS along with the Doctor helping him is designated with the proper term of...
...Companion.
The Doctor is the main character of the series but it doesn't mean that the companions are relegated to an unimportant level.
Several companions are as popular as any regeneration of the Doctor.
And each story is written choosing carefully not only which Doctor is the most appropiate but also which companion(s) fit better too.
This is a very remarkable reference book since the author not only cover the companions shown on TV but also on other media, primarily novels and audio dramas but also comic books and even short stories on the official Doctor Who magazine. Since 1963 with "An Unearthly Child" to 2013 with "The Name of the Doctor".
The relevance of the companions in Doctor Who has been even greater than the Doctors in the sense that you know that "The Doctor" is the "main hero", so he can't die (and even if so, he will regenerate (that cheating chap!)), he will do the right thing, etc... However, the flexibility of the companions is that some of them can die in shocking ways, some of them will leave, sometimes they will return, sometimes they won't, some of them can make huge mistakes, some of them will do what that even The Doctor doesn't dare to do, some of them can be from the past, some of them can be from the future, some of them are humans, some of them are aliens (including Time Lords), some of them are robots, even someone can become a sentient TARDIS!!!
The companions became so popular that even some of them got their own series on TV and/or prose novels.
I think that each Doctor Who fan has their own "favorite combination" of doctor and companion(s). In my personal case, it's the Fourth Doctor with Romana II and K9.
However, I like all the wide legacy of 50 years of the franchise, so in the same way that I like each doctor and their particular uniqueness, I like many, MANY companions too.
I love the reading experience of the book, I rated it with 4 stars (and not a full 5) due two reasons:
1.While the "classic era" of the franchise was done focusing quite well to the really key moments of the companions, I felt that the "new era" could use some editing to shorten it and reflecting a balanced/similar lenght of those chapters compared with the "classic" ones, sure on the "new series" you get to know a lot more of the personal life of the companions, but still I think that some editing can be achievable to avoid overwhelming the reader.
2.An omission of a companion... I wouldn't mind if some companion from other media than TV, such as comic books or audio dramas may be not mentioned here, but the book didn't cover "Astrid Peth" (Kylie Minogue) that appears in a TV Special and both "Wikipedia" and "TARDIS Wikia" defined her with a status of "companion" of the Tenth Doctor, so I think that a couple of lines, at least mention her, it couldn't hurt anybody.
However, don't get mistaken, this book is by far, the best option in the market for a reference book specialized on the companions in Doctor Who's franchise.
An extraordinary book indeed!