This is a worthy and well structured examination of the role of the Ministerial Private Office in the UK governmental structure. It is based on a PhD thesis and thus has the careful, signposting style of a good thesis. It is academically rigorous but not overburdened in that regard. So the generalist reader will be able to engage with it, although I suspect that the book will be of greater appeal to students of politics and government. The evidence base is extensive.
Although the book says that it is looking at the Whitehall Private Office, it is mostly about the Prime Minister's private office. In that respect, it does not give any new revelations for those who are familiar with recent British political history. The accounts of the Private Offices of Wilson, Thatcher and Blair are well known, and those who have read, for example, the works of Anthony Seldon and Charles Moore (on PMs generally and Baroness Thatcher respectively), will not be surprised at this account.
I think that Alun Evans misses a trick by not exploring further in his general analysis, the necessary qualities of a private office working effectively with its department and its "stakeholders" to enable them to support Ministers. By focusing on "No. 10", the book looks at relations upwards and across, not downwards.
Apart from that, and the need for some tightening of the book (e.g. there is some repetition e.g. several explanations of the French "Cabinet" model), I recommend this book to those who are interested in the role and potential benefit of a well functioning Prime Ministerial private office.