Oh my word, what have we here? A fat hardback, stocked with essays on the music, 18th-century diplomacy and the history of the house of Hanover? Stagy photos of Cecilia Bartoli as a shaven-headed cleric and a rather glamorous spy? Adverts for a documentary DVD to come, as well as a game for your iPad? My tip is to listen to the CD before you bother with any of this harmless fun, for in the end it will stand or fall on the music it contains. And that, as it happens, is seriously interesting.

Agostino Steffani (1654-1728), a composer, bishop and diplomat, is surely one of Baroque music’s most undeservedly neglected figures. A demonstrably important influence on Handel – who would have met him in Hanover, where Steffani was an Apostolic Vicar – he is known, if at all, for his elegant vocal duets, yet also composed about 20 operas of his own. The two dozen arias on this CD include no fewer than 21 world premiere recordings.

There is not room here to do justice to the personality and variety they display: suffice it to say that they range from the heroic to the tender, the loudly celebratory to the intimately tragic and the humorous to the deeply touching with all the ease and sensitivity of a natural opera composer. Like the arias of his contemporary Alessandro Scarlatti, Steffani’s can be short and pithy, yet his melodic writing is more shapely than Scarlatti’s, and sometimes even more so than Handel’s. Steffani’s music also reveals familiarity with the French style of Lully, something else which he may well have passed on to Handel. But enough now of Handel – these exquisite little arias have charm, colour and depth enough to win hearts on their own.

Cecilia Bartoli is to be both congratulated and thanked for this project, which appears to be very much a personal labour of love. Her dazzlingly virtuoso and urgently expressive performances betoken nothing less than total commitment, with every single aria delivered with as much dramatic intensity and focus as if it had been lifted straight from a fully staged production. With spirited accompaniment from the ever-vigorous I Barocchisti and Philippe Jaroussky joining for four duets, it is hard to know what more one could ask for. Unless it be a tie-in Donna Leon mystery inspired by Steffani’s life – oh hang on, there’s one of those as well…