Jenny Balfour Paul is an indigo dye expert. She has written two books on the subject, and lectures around the world. A librarian alerted her to the mention of the colour, and the plant it comes from, in the journals of a long-forgotten sailor and indigo hand. That day a ten-year love affair began.

Thomas Machell was born near York in 1824, a son of the manse. At the age of 16 he went to sea, scrubbing the decks of a merchant ship. After numerous adventures he settled in India, initially working for the Bengal Indigo Company, then transferring to plant and harvest coffee in Kerala. He was a curious, observant man who became fluent in both Arabic and Hindi, and, unlike many servants of the Raj, travelled widely off the beaten track. He crossed desert ridges from Suez to Cairo on a camel, for example, pacing first beside the new telegraph line and then the Nile. In her subtitle, Balfour Paul goes so far as to call him an explorer.

She quotes judiciously from Machell’s five volumes of journals, stabled now in the British Library. They are a delight, as the subject matter ranges from tooth extraction to the meaning of life and the ennui which frequently assails the author as he scratches with his quill by candlelight, flapping mosquitoes away from the inkpot. He lost his heart to a Polynesian beauty, but never married; from the journals one deduces that he was a sentimental chap. ‘I looked up at the glorious stars shining as brightly as the sparkling eyes of my lost Whyheva,’ he writes as his ship weighs anchor and his girlfriend’s island shrinks into the blue. Robberies, cholera, loneliness — it’s not all plain sailing. Whenever Machell does go home to England, someone else in the family has died.

Both the journals and Balfour Paul’s prose conjure the life of an Odysseus in India in the mid-19th century: the whiff of cheroots as a punka wallah tugs at the fan ropes, the bitter taste of a green coffee bean, the sound of dhows creaking in a monsoon breeze. Deeper than Indigo is a colourful, entertaining account. Machell illustrated his journals with sketches and watercolours, many of which are nicely reproduced here.

Balfour Paul weaves strands of autobiography into the fabric of the book — youthful overland trips to India, her marriage to an ambassador, her global travels in pursuit of Machell, including a stint in Texas communing with a descendant. She adds her own winsome illustrations to those of her hero, and the two styles complement each other.

The prose is solid, the tone whimsical (‘Picture the scene . . .’). Occasionally the author strikes a bathetic note: ‘Alexandria makes Exeter seem awfully dull by comparison.’ At the outset she refers to ‘meeting’ Machell, continuing to develop the relationship until they are writing letters to one another (at one stage she even sends him a fax). ‘Thomas gradually replaced indigo as the main passion in my life,’ she says. ‘We learnt to tango in time’; and she describes herself as ‘besotted’.

When she visits the Machell ancestral seat in the north of England she dresses up as him in a floppy-sleeved shirt, shabby velvet breeches and a wig. Most biographers start off thinking they will possess their subject, but in the end the subject possesses them — it has happened to me. But few go as far as Balfour Paul. In the end she undergoes ‘past life regression’, and has an out-of-body experience in which she converses with Machell. (This mystical experience takes place in the Malabar Hills. One thinks of Adela Quested.) This might be your cup of chai, but it’s not mine.

That said, this is a curiously compelling book. The indigo connection and Balfour Paul’s deeply personal passion give an authenticity to the quest that other ‘in the footsteps of’ books lack.

Thomas Machell died in Narsinghpur in central India, in 1864, aged 39, and the story ends with his dauntless biographer at last uncovering his grave. This book is his memorial.