The second problem with the book is musical. Take Die Meistersinger. Kohler is an up- to-date guide for the ways in which Wagner always used people and things, including the way this opera uses historic sources for Hans Sachs and medieval guilds. For although Kohler's plot summaries leave things unsaid, they are still generous, making clear in Meistersinger Wagner's burgeoning German nationalism and -though this is more complicated -the use he made of his emotional experiences.

But authors easily forget that whatever thoughts and feelings composers might have, they are intensely (primarily?) interested in music, what has been done with it before, what it can still be made to do.

So, while Wagner's large-scale application of medieval bar form (AAB) in Meistersinger is well known, there are many other significant details. For instance, the famous combination of themes in the opera's Prelude: what is this but an allusion to traditional German quodlibets, pieces combining familiar songs? (A then-recent publication had showed how five of Luther's Catechism chorales can be combined.) Or take the opening scene, as hero and heroine ogle each other in church: this is a wonderful record - in fact, the best we have -of how chorales were sung in Lutheran Germany, slowly, with interludes between the lines. Of course, Wagner does it on a bigger scale than usual, but he grew up with this kind of thing in Dresden and knew all about it. The chorale itself is an immensely skilful imitation of the real thing, and mysteriously original.

So is the apprentices' music, full of the little patterns used and taught by countless German kapellmeisters of old.

Such details are not the work of a merely self-indulgent, vain, deceitful and dictatorial scoundrel. Wagner was briefly a pupil of the Cantor of St Thomas, Leipzig, where he was baptized in 1813 (several months late, thanks to Napoleon), and although Kohler mentions some of this in passing, he does not point out that Wagner's radical reworking of harmony was anticipated in some of Bach's music, and had lain dormant meanwhile. Even the trumpet fanfares during Meistersinger's magnificently rowdy, chaotic moments (how can one even conceive such sound-worlds?) resemble the nightwatchman's calls Wagner, and Bach before him, had heard echoing in the streets of Leipzig. Never mind whether Eva stands for Goethe's Eternal Woman and Wagner's Unobtainable Ideal. Never mind Hans Sachs's rallying cry (unnecessary in Kohler's view) about True German Art.

Forget what this came to mean to Hitler, and especially to dissidents in the socialist DDR. The man was a musician, with a musician's paramount interest in the technicalities and practicalities of his art, even, so I suspect, using a piano to work out his revolutionary harmony.

My third problem with the book concerns what music is or does. Kohler's view, often impressionistically worded, is the one usual today: music is a sound world that conveys (expresses, articulates, evokes, invokes, etc) non-musical things, such as the listener's emotions, the composer's emotions, his experiences, his personality, cultural ideals, social function, national hegemonies, and goodness knows what. The book's careful narrative of both the life and the works is useful for making this case, and Wagner asks for it, having produced long-winded theories himself, expressed in that extreme, even violent way in which he seems to have expressed all his thoughts and feelings, on paper and in person.

Yet Kohler treats this approach to music's meaning not as something period bound but rather as a fixed truth, albeit one that can never be fully consistent. For Wagner, the link between art and life follows on the Hegelian Idea; or, in Kohler's words, that art had a universal function, proclaiming the hidden truth of existence by penetrating the mists of transient everyday life and shedding light on the sempiternal ideas.

To sceptics like me, this is risky ground and brings to mind a very funny cartoon in an early Punch (same period as Die Meistersinger) in which the Eager Young Lady at a concert is saying to a bemused gentleman, Oh, Mr Robinson, does not it ever strike you in listening to sweet music, that the rudiment of potential infinite pain is subtly woven into the tissue of our keenest joy?