Church guidebooks almost always get it wrong. The best are probably the unassuming ones, where the writers know that you are granting them four minutes of valuable time, and so get straight to business. Clock the marble font, the organ casing, the Saxon window, the bench ends, the 17th-century effigy. There, you've seen our greatest hits; thanks for your attention, and don't forget to put something in the box.

Church guidebooks almost always get it wrong. The best are probably the unassuming ones, where the writers know that you are granting them four minutes of valuable time, and so get straight to business. Clock the marble font, the organ casing, the Saxon window, the bench ends, the 17th-century effigy. There, you've seen our greatest hits; thanks for your attention, and don't forget to put something in the box.

More often than not, there is a booklet written by a vicar in the 1930s, stored in a damp cupboard since then, that tells you which bit of the north wall was built in the 1300s, who founded the almshouses across the road, and when the underfloor heating was put in. More recently, guidebook writers have been asked to include something about the purpose of the building, and so now we are told that pointed arches direct our minds heavenwards, and would we like to join in the Lord's Prayer printed on the back?

Margaret Visser - historian and anthropologist, Roman Catholic and traveller - knows churches and church guidebooks. Fascination with the first and exasperation with the second prompted her to start this project: a detailed description of a single church, and of the faith that has shaped it down the centuries.

Fortunately for the reader, Visser's subtitle, "space, time, mystery and meaning in an ordinary church", is inaccurate. Sant' Agnese fuori le Mura, just outside the old walls of Rome, is an extraordinary church. Not particularly big, but old, with its roots in the fourth century. It is built over the catacombs, incorporates Roman pillars, draws in a nearby rotunda, Santa Constanza, which was built as a mausoleum for Constantine's daughter, and grew out of a huge cemetery-basilica discovered just 50 years ago. More particularly, it houses the bones of St Agnes, killed in 305 for refusing to compromise her faith. (The larger Sant' Agnese in the Piazza Navona holds her skull.)

Visser's technique is to take the reader on a virtual tour of the building, beginning with the entrance, working through the nave towards the altar, wandering across to the side altars, snatching a breath of fresh air outside, and ending up at Agnes's tomb. Because this is a virtual tour, she need take no heed of swollen ankles, and so stops the party every paragraph or two to discourse on each item, whether an architectural feature, a liturgical practice or a spiritual bon mot.

This can be fascinating and tedious by turns, the latter because she assumes no knowledge in her target readers, whom one suspects to be largely Americans. Thus she feels the need to explain how to play hopscotch and that churches are thought of as ships.

The greatest test of Visser's ability to hold on to her reader comes in the passages about the eucharist. One description worked for this reviewer: "In spatial and temporal terms, always and everywhere become, in the eucharist, one with the here and now." One did not: "Eating, even before sex, is biological evolution's first step towards transcendence in the animal species because it initiates physical openness to and need for the Other." Time to dive back into the history of another part of the church, which the author mercifully does. And this rocking back and forth between observation and insight works, on the whole. If a particular nugget from Visser's research does not appeal, skip.

Two regrets, the second more serious. First, I must presume the publishers decided not to include photographs of Sant' Agnese. If the book's purpose is to be inspirational as well as informative, this strikes me as a blunder. Visser remedies this, in part, with some snapshots on her website at: www.margaretvisser.com.

Second, there is little sense of the present congregation: no names that I can recall, no description of the type of person drawn to worship regularly, and therefore no conception of the church as a group of believers, which is the first and truest meaning of the word. Visser describes briefly a side chapel got up as a Lourdes grotto, and writes, dismissively: "To most people... it does not matter what it is that engages and concentrates the mind, just as long as it does the job."

But this is, surely, the book's main thesis. Visser has painstakingly pieced together the religious use made down the ages of a humiliated and murdered fourth-century teenager, but fails to turn her keen gaze on how it fits with the piety of present worshippers. It is a pity that the anthropologist nods, and allows the historian-guide to raise her furled umbrella, and to ramble on.

Paul Handley is editor of the 'Church Times'