Doctor Who: Companions is a coffee table style book covering the companions of the entire original series 26 year run. I've had the book for a long time but only used it as reference because it is too large to easily carry around. There are many pictures with captions of the actors both in and out of their characters. The written material is surprisingly interesting and is thought provoking enough so I found it not good sleep time reading. The actors playing the Doctor are only mentioned in passing but there is some interesting information.
For each companion the authors cover; the development of the what was needed in the show at the time, how they were cast, the success from the writers standpoint and why the character left. The actors point of view is given on the casting, problems and successes during their time, how they enjoyed job, and along with relevant anecdotes. The book ends with a chapter on other media such as plays, movies and books after the series ended. The afterwords lists each character alphabetically and briefly describes the actor's career.
Even for experienced readers I would recommend keeping a chronological table of the titles, doctors and companions handy. The only one I know of is at SciFiOne.com but there might be easier ones to use.