His most recommended book would be his unconventional biography of Louis XIV. It is a long and detailed book, yet instead of focussing too much on the minutiae like nearly all biographers of Louis XIV did before him, approaches the reign from a global approach aiming at a synthesis of the man and his time and make it comprehensible, and ends up in the process drawing a very good portrait (if of course debatable - and of course other historians did) of France under Louis XIV - social and political structures, economy, the arts, everything goes. It's all the more interesting that, unlike most French historians (not as true with contemporaries as it was a few decades ago, but still), he did take into account for his research the thesis and works of the most prominent non-French specialists of 17th century France. Some of the best specialists of Versailles or its system today are British (especially on the economy aspects) and Americans (analysis of the working of the Court, an excruciatingly detailed history of Versailles itself) who often work and publish in French.

Petitfils has notably one of the best synthesis available to non-historians of the role of the arts in the Versailles system. It owes heavily to specialized works - (notably to a rather magnificent volume on Lully (and by natural extension, Molière) which I have, but it's quite specialized and I found Petitfils summarized its key elements and integrate them in his own global picture of this theme really well.

And well, he is rather entertaining too.

A lot of his books, all of them focused on the Bourbons, from Henri IV to Louis XV are really quite good. Beside the one of the assassination of Henri IV, he wrote a great essay on the real D'Artagnan, and a quite interesting study on Le Masque de Fer, and another very interesting, myth-busting study of the gruesome Affaire des Poisons.

The other biographies I read from him are also all pretty good (if none as stellar as his Louis XIV).. Montespan, La Vallière, Louis XIII, Louis XV, Fouquet, are the ones I've read so far.

Aside from the book on Henri IV, he wrote an episode of a documentary TV show on that topic. Each episode showed an hour by hour account of a single historical event, a kind of twist on 24. Not as good as the book (no surprise there), but quite watchable.