"Ahistory" is the latest edition of Mr. Parkin's attempt at chronologizing Doctor Who. As such it is a humongous work with seemingly endless entries about every tiny nuance that ties individual episodes of Doctor Who together, along with the books and audio adventures of the same.
The trouble here is that everything is an enormous mess, because nobody really cared all that much about continuity in a show that was originally designed to be shown once and then taped over.
What makes it worse, according to "About Time" (which I recommend instead), Mr. Parkin seems to have actually written Doctor Who novels to cover plot holes in the continuity (such as Tegan not liking transmats when she'd never seen them before in the show -- surprise, there's a story he wrote where Tegan encounters transmats!)
There is much in this book. Far, far too much. Also it contradicts many things that are said in "The Discontinuity Guide" and "About Time". It even contradicts itself in places.
One gets the sense that it's all a bunch of fanboys arguing with one another, and not a respectable history (or even ahistory) that tries to be definitive.
So this is a terrible work for someone wanting an introduction to Who, but is great for those who want to write their own Who and would like to know what happens in the Somethingth Century so they can put their story there.
The major redeeming feature is that "The Discontinuity Guide" and "About Time" do not cover the books or audio adventures to any real extent, and "Ahistory" does. But even this can be seen as a handicap when there is still much debate over the canonicity of the books or audio adventures.
A purist will probably go for just the television series, as there is enough of that to last a lifetime (28 seasons so far), and will likely wish to pass on this book. But the rest of us; the completists, the people interested in what the novels are saying without wishing to actually read them, and the novelists and writers of fan fiction; these will want to have a crack at "Ahistory".