Afterwards, in the back of a sleek Mercedes speeding towards Altenburg, Gardiner – often referred to as JEG – talked to me about JSB. This, as Gardiner effusively made clear, was someone who was very much a fellow human being. ‘He’s flesh and blood. Wonderful! I don’t believe in hagiography. I don’t believe Bach was a saint. He was a great guy and a great composer.’

That great-guy-great-composer mantra has stuck in my mind ever since. Why shouldn’t Bach be as human as the rest of us? Mozart, after all (as Salieri whined in Peter Shaffer’s Amadeus), had the composing genius of a god wrapped up in the blotchy exterior of an obscene boy. So there must be a hitherto unnoticed flaw or two somewhere in JSB’s glass.

‘Bach as Mensch continues to elude us,’ writes Gardiner in his preface to Music in the Castle of Heaven. Indeed, Gardiner’s search for the inner Bach is so special because it is as much a musical and personal quest as a biographical one: another pilgrimage, but one in words rather than music. You will have to look elsewhere for a complete analysis of Bach’s life and works. As JEG puts it, ‘My focus is on the music I know best.’

In a way, Gardiner’s narrative is as much about himself as it is about Bach. His effervescent writing style is the literary equivalent of his own performances: lively, intense, and designed to go straight to the heart of the music without dithering or pretence. Gardiner grips one from the start, and how could he fail?

‘I grew up under the Cantor’s gaze,’ begins chapter one – as he did. The young Gardiner’s early years in the Dorset farmhouse in which he was born were supervised, so to speak, by the stern presence of one of two portraits of Johann Sebastian Bach by Elias Gottlob Haussmann. This masterpiece, now housed at Princeton, was the property of a Jewish emigrant, Walter Jenke, who fled Germany in 1936 with the portrait and a guitar. He left it with the Gardiner family for safekeeping.

As a child, Gardiner found Bach’s gaze ‘impassive and slightly forbidding’; it was only sixty years later, when he visited the portrait at Princeton, that he could discern the ‘serious and sensual’ side: from the eyebrows (brushed in the wrong direction, he notes) and the slightly droopy eyelids (a footnote – one of many witty and perceptive asides – ascribes this to an hereditary eye disease, blepharochalasis), past the flared right nostril and down to the fleshy lip and jowls ‘that suggest a fondness for food and wine, as the records imply’.

The objective of looking deeper into Bach’s eyes is, as Gardiner points out, the best way to establish a holistic view. Or, as he says,

To see Bach in the round and not through the parochialism of small sampling or the microscopic perspective of single issues that some scholars defend with terrier-like tenacity. It has meant balancing musical analysis with broad historical contexts and establishing how his being in a particular time and place located his achievement in the wider development of European culture and currents of thought.

At the centre of Gardiner’s extended narrative is how Bach worked so quickly but methodically, particularly in the back-to-back writing of the stream of Leipzig masterpieces from his small composing room in the Thomasschule. From this noisy and cramped little space, for almost thirty years until the end of his life, Bach in his creative fury turned out not only his annual cycles of cantatas but the two astonishing Passions (Sts Matthew and John). The studio and its school do not survive; but the spirit of place so eagerly conveyed by Gardiner is the next best thing. Here, then, we find the shelves piled high with scores, on the desk the special manuscript paper (heavier than normal, to enable it to sit upright on the music stands), the five-nibbed music stave-ruler (rastrum: great for Scrabble), and pots filled with black, sepia, and red ink.

This, though, is merely the set-up; the process of active – indeed, over-active – composition was quite another. Bach had all the luck, Gardiner says, of being so fluent that he could compose directly into score, sometimes, while waiting for the ink on the page to dry, jotting a hasty tablature of shorthand in the lower right-hand corner.

J.S. Bach, 1750

What Gardiner – conductor as well as biographer – is able to tell us so perceptively and fascinatingly is how the cantatas flow from the page to the stage: a process he was all too familiar with from the Bach Pilgrimage, with its intense and cyclic schedule (Gardiner being a farmer must have helped, too). Here, in what he calls ‘creative chemistry’, the worlds of long-ago composition and the contemporary performance are brought together. It’s similar, he says, to baking a cake:

It begins with sieving and weighing all the constituent elements … but … you need to ensure that they all react organically to one another, taking responsibility for their respective roles and tasks. Because every musician is responding primarily to a single printed line of music that contains only his or her part, it is also the moment when each needs to switch on their aural radar and establish lateral awareness so that they find out rapidly how their line fits into the overall fabric.

Then, though, comes what he calls the ‘indefinable process’ that goes beyond technical accuracy to encompass ‘an enhanced sense of clarity or understanding’. Only then, Gardiner writes, can there occur a positive charge of understanding and involvement: ‘For if one can feel a connectedness of Bach’s music to this degree and intensity [at this stage], imagine, then, the force and potency of what it must have been like to experience the music in the hands of the person who created it and realised it first.’

So what of Bach the Mensch? Precious few personal anecdotes survive. What can be discerned, says Gardiner, lies right under our noses: the evidence that links music and mind and, if you look hard, reveals crucial aspects of the man.

Entwined in the music and situated behind these pieces’ formal outer shell are the features of this intensely private, multifaceted human being – devout at one moment, rebellious the next, deeply reflective and serious for the most part, but lightened by flashes of humour and empathy […] the music gives us shafts of insight into the harrowing experiences he must have suffered as an orphan, as a lone teenager, and as a grieving husband and father.

Music in the Castle of Heaven is an enlightening and personal quest into Johann Sebastian Bach’s life, times, and music. But words on the page, like all those notes in whichever coloured ink on Bach’s manuscripts, only tell part of the tale. Hearing is as important as reading. I suggest reading Gardiner’s book in tandem with his recordings. You can find the Bach Pilgrimage Cantatas available either separately or complete on 56 CDs. Either way, the rewards are boundless. JSB and JEG are both great guys.