The clear notes reveal spiritual movements

Alfred Dürr, a stalwart of the new complete edition of Bach's works, which was begun just after the Second World War, came to particular prominence as one of the two scholars who produced a "scientific" dating of Bach's cantatas in the late 1950s. "Scientific" in this context meant basing the chronology on the types of paper used and analysing the handwriting of the sequence of pupils acting as Bach's copyists. This replaced the traditional religious-biographical method by which, for instance, the great chorale cantatas were assumed to come from Bach's last decade, thus representing the apex of the composer's Lutheran faith.

What Durr startlingly proved was that Bach wrote the majority of his cantatas during his first five years at Leipzig, 1723-28, often at the rate of one a week. Suddenly, there were all sorts of calls to doubt whether Bach was really the great spiritual figure he had been made out to be; perhaps he was just a jobbing musician, crafting musico-spiritual aids on a more-or-less mechanical basis. All this changed again with the discovery of Bach's copy of Calov's Bible Commentary in the late 1960s. The annotations suggested that Bach was indeed deeply interested in religion, albeit more concerned with noting details of numbers and distances than with uttering deep spiritual statements. Bach thus remained both religious and supremely musical, but exactly how the two facets connected was by no means clear.

Dürr's guide to the complete cantatas goes some way towards showing how the music of Bach's cantatas ties in to its religious function (or, in the case of the handful of secular cantatas, to its political or social function).

It actually dates back to the 1970s, although the author has considerably revised and expanded it over 20 years. Now, at last rendered into excellent English and revised by Richard Jones, it has been updated yet further, although it remains indisputably the work of Durr.

Given Dürr's credentials as the ultimate forensic expert on manuscripts, it is perhaps surprising how accessible this guide is. It is absolutely reliable so far as dating and context are concerned, but the author has also made a genuine effort to describe the cantatas for the non-specialist.

Thus there is no better authority to which either the music scholar or the music lover can go for an introduction to any particular cantata.

The guide begins with a basic introduction to the history of the Lutheran cantata, together with its relation to biblical exegesis and free spiritual ("madrigalian") poetry and the music elements ranging from motet and concerto to the Lutheran chorale and the more modern forms of aria and recitative. Given that Bach titled the majority of his cantatas "concerto", it would perhaps have been desirable to have a more searching analysis of this principle (its historical meaning tends to range from opposition of musical forces to agreement between independent elements), but the overriding advantage (or disadvantage) of Dürr's enormous study is its even-handedness. Every element is given equal weight; each cantata is given more or less the same amount of space.

Next we learn of the development of Bach's cantatas, from the earliest, written in Muhlhausen, to the three great cycles from Leipzig. Dürr is characteristically cautious about the original existence of later, lost, cycles: he assumes that there was a cycle setting the known text of the Leipzig poet Picander but tends towards the view that Bach had an adequate supply of cantatas after 1729, since most later surviving cantatas tend to relate to special purposes.

Particularly useful is the short section on performance practice, which gives the basics on how the composer's performing material was formatted, information on the recent theory of one-to-a-part vocal performance, scoring and ornamental practice. The view that Bach's performances had "a decidedly improvisatory character" is tantalising, although perhaps too briefly stated here to be of much practical help.

The concluding remarks on performance unambiguously proscribe instrumental sounds that are "anti-baroque", though the author seems to skate over the fact that Bach's violone is a completely different instrument from the double bass (whether of Bach's time or our own).

The entries on individual cantatas are placed in order of the church's year rather than in the order of the familiar cantata numbers (the indexes help one to reconcile the two orders). Each begins with a statement of the Epistle and Gospel for the day concerned, the duration and then the parallel German and English texts. This is then followed by a substantial textual commentary.

To take the well-known cantata Wachet auf , ruft uns die Stimme , BWV 140, as an example, Dürr first examines the way it was written in 1731 to supplement the earlier chorale cycle and he then outlines the source of each line of text (much is taken from the notoriously erotic "Song of songs"). In describing each movement in turn he shows how, for instance, the underlying chorale melody influences some of the orchestral figuration and how the division of instruments prepares us for the dialogic nature of the cantata as a whole. He explains how the first duet - "among the most beautiful love duets in the musical literature of the world" - is enhanced with a violin tuned a third higher than a normal violin, perhaps to depict the glorious radiance of the bride (the church) awaiting her lover (Christ) "with burning oil". Next follow brief descriptions of the famous trio movement (later used as an organ chorale prelude) and the second duet, a cheerful, dance-like movement. The final chorale, with the added high violin, is read to "give symbolic shape once more to the bliss anticipated by the Christian in the heavenly Jerusalem".

Dürr adopts much the same format for secular cantatas, too, although one might feel that his heart is sometimes not quite in the task at hand. For instance, the "Coffee cantata", which is undoubtedly a work of considerable musical substance, "surely does not belong among the principal works of the Thomascantor"; nevertheless, Dürr admits that it suggests that Bach could have been a promising opera composer. The rest of the commentary contains some half-hearted descriptions of text setting and word painting.

In all, then, this is extremely reliable, "safe" description of Bach's extensive oeuvre of cantatas and oratorios (which, as the recent Radio 3 Bach marathon demonstrated, took up well over half of the ten 24-hour days devoted to the composer). The musical analysis is generally sensible, if unambitious, and the theological inferences are sincere but never extreme (no numerology or other cabalistic codes here).

The book is thus ideal for those who are already converts to the Bach cause; it is not likely to win new converts. It will not provide much enlightenment for those who seek to understand what makes Bach's music so special, or if one wants to ask thorny questions about how the music really relates to texts that are often patently absurd by modern standards, or how Bach could sometimes make the same musical material serve contradictory texts when he reused the music for new occasions.

On the other hand, there is very little writing on Bach that is truly critical or addresses cultural concerns. Within such a context, Dürr's cantata guide remains of outstanding significance, and anyone able to face the price tag of £175 can be sure that their purchase will remain authoritative for many years to come (and a £40 paperback is promised soon).

John Butt is professor of music, Glasgow University.