This is the raw personal notes of Ciano-- not an historical narrative. It is well supported, but the fact that it is a diary means that it doesn't present the complete picture. Ciano notes in several places, for example, that he has recorded the details of this event or that in the official records (which may or may not be available from other sources), then records only his personal reflections that amplify the official records. In some places the descriptions are terse, and it is clear that he recorded only keys that, in a post-hostilities frame, would enable him to recall the full record of an event.
The best parallel I can think of is Cornelius Ryan's interviews with Heinrici for _The Last Battle_, in which the general progress of events was based on the official record, on Heinrici's notes from the time, and on Heinrici's personal recollections during interviews with Mr. Ryan that actually filled in the details that the cold notations of the records didn't supply. In this printing, you get only the personal notes, not the greater historical perspective or the personal recollection that would fill in the blanks-- since Ciano was shot for opposing Mussolini in the end. The book is often cited for the insight to Mussolini that it provides, but because Ciano as Foreign Minister was involved in many areas of Italian government, it provides much more value than that.
I think the diary gives unique insight for people who want to understand why Italy was such a nonentity in the early years of the war; it doesn't elaborate it completely-- the question of why Italian aeronautical technology was so backward is not even mentioned --but it does present an upper echelon view, from the Italian perspective, of why their initial disadvantages were impossible to make up. Most importantly to me, why the Italian elite could understand that their national objectives were best served through cooperation with the Allies, but their survival required alliance with Berlin, is clearly drawn out here. The humor of the German battle plan as compared with the tragedy it imposed on the rest of the world is likewise just under the surface to the educated reader. The diary further enumerates the infighting and more detailed balances required to manage creating an empire from a country that was nearly prostrate to start with.
It requires discipline to read, as do almost all published diaries, because it isn't finished prose-- it's the notes that a top-level official jotted down at the end of the day to remind him later of what happened that day. However, it provides a unique view of the politics of governing Italy and conducting foreign policy during the years 1939-1942. As Ciano is a key figure during most of time, it provides insight to the southern side of the ETO, mostly of the populations and personalities involved, that isn't easily available elsewhere.
For Ciano himself?