The world's first opera - Peri's `La Dafne' - made its debut 400 years ago this month. Since then the number of titles has grown well into the thousands. Antony Peattie, co-editor of `Kobbe', the opera-goers' bible, explains how, for the latest edition, he and Lord Harewood narrowed the choice down to just 499.

Gustave Kobbe's The Complete Opera Book was first published in America in 1919, the year after its author had been killed in a freak boating accident off Long Island. It was never, can never be, "complete", since choices have to be made. Even with extra entries added by Ferruccio Bonavia, the original edition still covered only 110 operas in detail.

Since 1954, Kobbe has been through three revisions under the editorship of Lord Harewood, former managing director (later chairman) of English National Opera and founder of Opera magazine. His last (1987) edition covered over 300 operas. The latest edition, co-edited by Lord Harewood and myself, drops the word "complete" - it's now renamed The New Kobbe's Opera Book - but covers 499 titles. It omits rather more, of course, including some 50 by Vivaldi, 60 by Alessandro Scarlatti... Further omissions will irritate those with a copyright axe to grind ("What, no Respighi?"), cultists and excessively broadminded bien-pensants. You can have hours of fun going "Tsk, tsk".

Kobbe's own style was marked by conviction: he believed in his choices and did not hesitate to enliven plot summaries with personal reminiscences of performances: "I was a boy when, more than 40 years ago, I first heard Fidelio in Wiesbaden. But I still remember the thrill, when that trumpet call split the air with the message that the Minister of State was in sight and that Leonore had saved her husband."

On the other hand, when the Grail is disclosed in Act 1 of Wagner's Parsifal, Kobbe claims that it "shines with a soft purple radiance that diffuses itself throughout the hall": having witnessed the opera's premiere staging at the composer's own festival theatre in Bayreuth on 26 July 1882, he assumed that its effects would be reproduced at all subsequent performances.

A native New Yorker, Kobbe's selection was largely dictated by the repertoire of New York's Metropolitan Opera. A fascinating document of its time, Kobbe found room for six operas by Meyerbeer and as many by Mascagni. Italian opera dominates the lists, with now generally forgotten works by Zandonai, Leoni, Mancinelli and Franchetti. Where now are Erlanger, Bruneau, Fevrier, Xavier Henry Napoleon Leroux, Gunsbourg, Nessler, Thuille? As for "new" works, where are such novelties as Frederick Shepherd Converse's Sacrifice, John Adams Hugo's Temple Dancer or Charles Wakefield Cadman's Shanewis? When we looked at contemporary operas, Lord Harewood and I agreed that, to qualify for inclusion, they had to have been given an independent, second production, since premieres are so often occasioned by pious, political reasons (eg the composer is Welsh; it's the Columbus quincentenary). Like all the best rules, it was there to be broken, and I was relieved when Lord Harewood assured me that he thought consistency "an over-rated virtue".

Of the Mozart operas, Kobbe originally featured only Figaro, Don Giovanni and The Magic Flute. Bonavia added Die Entfuhrung (Italianised as Il Seraglio) under "Recent and Revived Operas" - squeezed between Messager's Madame Chrysantheme and Rimsky-Korsakov's Ivan the Terrible. Cosi fan tutte scrapes in later still: "The Mozart revival has called attention to this slight but delightful opera". But the courage to make such judgements is unique to Kobbe. It still distinguishes it from such heavyweight rivals as The New Grove Dictionary of Opera (four volumes, 1992, pounds 550) and The Viking Dictionary of Opera (one volume, 1993, pounds 60), which aim at completeness, use many different experts and eschew personal experience of performance.

As the editor of Kobbe, Lord Harewood has brought to bear on the entire repertory his vast, unparalleled experience, gained in senior roles not just at ENO but also at the Royal Opera, the Edinburgh Festival and Opera North. I was brought in after the sudden death in June 1996 of Nicholas John, ENO's dramaturge. He was due to re-edit Kobbe and I'm sure it would have been remarkable: he planned to cover Hasse (1699-1783) as well as Graun (1703-1759). Before he died, however, he had completed only three, very long entries. Having attended the same school and university as him, I went on to do much the same sort of job (publications editor at Welsh National Opera). I wish we could have debated Kobbe together. I think we might have agreed on one thing at least: because it dares to select from all that is available, Kobbe can usefully guide readers towards operas they may have missed.

The performing repertory in Britain shrinks as decreasing subsidy leaves companies fearful of experiment. Opera North currently leads the field in regularly encouraging its audiences to try the unfamiliar: it has, over the years, given the British stage premieres of such rarities as Schreker's Die ferne Klang and Gerhard's The Duenna. Recently I caught the company's new staging of Martinu's Julietta on tour in Hull, where it reached a large and enthusiastic audience in a week that also included Cosi and Aida.

Covering opera's murky waterfront for Kobbe makes one passionate about many comparatively unknown pieces: Lord Harewood and I both now carry torches for a whole range of recorded but rarely staged works - from Schubert's Fierrabras and Sallinen's The Horseman (H.) to Rameau's Zoroastre and Strauss's Guntram (A.P.)

But my time at WNO taught me that, even when it comes to more "standard" repertoire, British audiences increasingly long to get more out of their opera-going than mere passive pleasure: hence the emergence of intelligent programme books, Nicholas John's admirable Opera Guide series and pre- performance talks. But, unless you belong to the minority who can read music, reading about music often frustrates as much as it enlightens. What was needed, I realised, were taped talks about operas that would tell you about the plot and the music, including extracts from recordings. The same thought had occurred to Katie Tearle, head of Glyndebourne Education, and (thanks to sponsorship from the Vivien Duffield Foundation) we have now made 15 Opera Bites cassettes, covering four operas by Handel, four by Janacek, six by Britten, and two each by Rossini, Mozart, Rameau, Martinu, Strauss, Berg and Verdi. All the works are being, or will be, staged in Britain - except for Rameau's masterpiece Hippolyte et Aricie, the obvious "B-side" (hint, hint) to Platee. To write the scripts we asked experts who could communicate their enthusiasm, such as John Tyrrell, Richard Osborne, Michael Kennedy and Jonathan Keates. As a result, you can now stick one in your car stereo on the way to the theatre and hear how Janacek was inspired by Madam Butterfly in Katya Kabanova; how Rossini used an 18th-century ballad tune to portray Count Ory; how Handel turns a chorus into a congregation in Theodora; how Britten ransacks American popular idioms in Paul Bunyan and so on. The readers include Fiona Shaw, Eleanor Bron, Timothy West and Kathryn Harries. We called them Opera Bites to emphasise that they are accessible. Our greatest challenge will come next year, when we try to make Opera Bites for works that have not yet been performed...

The `New Kobbe's Opera Book' is published by Ebury Press at pounds 45. `Opera Bites' are available at pounds 6 (incl p&p) from 0118 978 9303