(Please note that Doctor Who Encyclopedia (New Edition) has now replaced the edition I originally reviewed. This is a good thing.)
I suppose one could use Doctor Who: The Encyclopedia to actually look things up, but really, I think it's more fun to start with the letter A and peruse its entries in order. This ensures that you don't miss any of the goodies within, while providing almost as chaotic a tour of the worlds of Doctor Who as one might get in the TARDIS itself.
Take the letter D, for example. Here you learn why Madame de Pompadour was once called Madame d'Etoiles, what newspaper the Absorbaloff was reading when Ursula noticed his alien hands, the name of the beach where Rose said goodbye to the Doctor, the human name of the Slitheen in charge of the North Sea Boating Club, the provenance of the Doctor's claim that Martha was from Freedonia, the name of the Cabinet minister who called Saxon "insane" - and, of course, quite a bit about the words "Doctor" and "Daleks." The above list doesn't cover even half of the D entries found in the book, ranging in length from a sentence to an entire page each.
Doctor Who: The Encyclopedia covers the first three seasons of the 2005-present revival of the series, plus the occasional mention of real-world creators of songs, films, etc. that appear in the show. (The new edition more than doubles the period covered, taking us into the Eleventh Doctor era.) I imagine the book could prove invaluable for the novice Doctor Who fan in learning more about the show's characters and continuity. Even a serious Doctor Who addict such as myself will find nuggets of forgotten or unsuspected detail, such as corporate information gleaned from a reception guest's telephone, or character names that were never actually heard on screen, or simply went by too fast.
To keep the book to a manageable length (and possibly for licensing reasons), virtually nothing from the 1963-1989 adventures is mentioned unless it has been referenced in the new series, in which case a few older details are sometimes included. A few entries are misleadingly incomplete as a result. Aside from some awkward or repetitive wording, this is my only quibble with the book. I'm glad I bought it, and will almost certainly buy it again in the new edition eventually. Technically I've already done so, as an app for my iPad, plus supplements.