This wonderful book tells the lives of the twin sisters Agnes Smith Lewis and Margaret Dunlop Gibson. The motherless daughters of an Ayrshire lawyer who by chance became one of the richest men in Scotland, in childhood they were given the opportunity to visit any country, so long as they learned the language first. This gave them a footing which enabled them later on to master modern Greek, Arabic and then Syriac. Travelling first to Egypt in 1868, they made many further visits. Both married in their forties, and both were widowed in three years. Agnes’ marriage to the librarian of the Parker Library at Corpus Christi College, Cambridge, led them into manuscript studies, and so to the famous visit to St Catherine's Monastery, Mount Sinai, in the course of which Agnes found the palimpsest manuscript of the Old Syriac Gospels, the Sinaitic Syriac. They subsequently became collectors and editors of manuscripts. They acquired a fragment of the lost Hebrew original of Ben Sirach, a find which led Solomon Schechter to the Cairo Genizah. Among many generous acts, they provided funding to build Westminster College Cambridge.

The book is funny (I gained disapproval by laughing out loud in the quiet carriage of the 16.22 from Paddington), sad (in the stories of their bereavements and Agnes’ last years) and vivid both in the wider vision and in the details with which it interprets the bare events, from the small-town church life of Irvine in the mid-nineteenth century, to guidance for Nile tourists in the 1860s, to the Cambridge of the last decade of the century. No less vivid are the people whom the reader encounters: Rendel Harris, Father Galaktéon of St Catherine's and Solomon Schechter (all generous friends to them); F. C. Burkitt and R. Bensley (considerably less congenial); William Bruce Robertson (their minister when they were young) and Robertson Smith (another good friend).

Neither the excitement generated by the discovery of manuscripts nor the quality of scholarship needed to edit them should be underestimated. The degree of interest which the Greek manuscript of the Bible Codex Sinaiticus continues to elicit means that we should not be surprised at the celebrity status which the twins came to enjoy, while their sheer hard work in finding, studying and editing texts explains why they were given honorary degrees by universities including Halle and St Andrews.

The book is far more than a portrait of two remarkable people. With effortless ease and without the reader noticing, it is much more, drawing us into many important issues. One may single out the storms of nineteenth-century theology; the relationship between traditional Christian belief and the new finds of textual scholarship; the struggle for the right of women to take degrees; the relations between northern Europe and the churches of the Middle East; the development of Presbyterianism in traditionally Anglican Cambridge.

Nor is it the least of the book's achievements that the human story is told so sympathetically and warmly. In particular, the rows that developed in the party that went to transcribe the manuscript at St Catherine's (the twins, Rendel Harris and Burkitt and Bensley and their wives) are well told. The characters emerge through perceptive observation, not least of their portraits (Agnes at the age of 14 with ‘the serious face of the senior twin’, or Burkitt's wife as ‘a perfect belle époque beauty’).

This is a very accessible book (it was serialised on BBC Radio Four), but with thirty-two chapters and over 300 pages it also expects us to take it seriously. We are used to thinking that serious books are written in an obscure way and that heaviness shows intent. This book, with its sureness and lightness of touch, shows us another way. To prove just how good a good book can be, this one should be on every reading list.