I like my biographies to make a clear distinction between fact, opinion, supposition and downright fantasy. They should also follow a clear line, preferably chronological. This biography is not one of those.
Though the facts are all here, they are presented in a somewhat confusing manner, with a tendency to flit around from person to person and from year to year, which often negatively affects the clarity of the narrative. The author's opinions getting far too much emphasis. She presents herself as the only arbiter of truth, in the manner of "x says that y was Christie's best book, but in fact it was not - z was". Quotes from many of Christie's books have been mined to support suppositions, and it becomes weary reading at times. A surfeit of "of course"s and "in fact"s grate throughout - my comparitively high rating is an acknowledgement of the academic depth of the book, bu this comes at the cost of readability.
It's also quite annoying and without any logic that the author describes so many photographs and of so many people, and that so few are reproduced in the book.
And yes, the author reveals the ending of too many of Christie's books. Not bad in itself, but the author does this without warning, so don't read this unless you've read all her books already or you have a short memory for such matters. It's part of a sort of projected arrogance throughout which, for example, assumes that the reader has read all of Christie's works already; that they know who all the people named are/were; that they need no explanation of acronyms and abbreviations; that they all speak French (foreign-language quotes are usually left untranslated) and so on.