What would Georgina Masson, author of The Companion Guide to Rome, and John Fort, who has recently revised her text, have said to one another if they had ever met? Possibly Fort might have asked her about her dog Willy; she in turn might have questioned him about sales of smoked salmon in his shop in Palazzo Doria Pamphilj. And then they would have moved on about their business, in that rather understated way of most tenants of the Doria family, who enjoy the security of having one of Romes old noble families as their landlord.

The two never met, and if they had they would probably have been surprised to be told that one day Fort would follow in Massons footsteps as he set about revising her guidebook. Both of them might have been equally amazed to discover that what was initially planned as another update (a previous one had been done by Tim Jepson in 1998) turned into something much more substantial.

When Fort agreed to revise The Companion Guide to Rome in 2001 he had little idea what was ahead of him. Although he has lived in the city since the early 1970s he admits that he hardly knew Massons Rome. He had never done one of what he describes as her gruelling walks until he was asked to revise a trial chapter. Most of his time had been spent on the busy delivery rounds for his smoked salmon business. This he conducted with his wife Mary out of their small shop on the corner of Piazza del Collegio Romano and Via della Gatta, and from Licenza near Tivoli where his uncle started smoking fish in the 1960s.

Soon after the Forts sold the retail business in 2001, their landlord and friend Prince Jonathan Doria Pamphilj suggested Fort might like to revise Massons book, and so began a year-long adventure which involved much more than updating information about museums and their opening times.

Fort describes what he has done as a rewrite. But what he has achieved is something much subtler, because even Massons greatest supporters would not be able to tell where Masson ends and Fort begins. The rewriter has followed the authors style with such skill that the text appears seamless, and it is only when the two versions are read side by side that it is evident what Fort has added.

Fort gives a clue to his real achievement in his revisers note at the beginning of the book. If Masson has introduced Rome to Fort, then Fort has introduced the Baroque to Masson. It is difficult to imagine how Masson, the 20th centurys equivalent of the 19th-century Augustus Hare, could have passed up so many of the citys baroque churches. It is also intriguing that those who frequently use her text as a reference book never complain about omissions. But compare the Masson and the Fort texts and you will find plenty.

The best examples are contained in the new editions chapters 10 and 11, S. Maria in Via to Via del Babuino and Piazza del Popolo to Piazza Thorwaldsen respectively, which correspond roughly to chapter 10 in Massons original text. Here Fort includes sections on the churches of S. Maria in Via, S. Silvestro and S. Andrea della Fratte. His description of this last church, in Via Capo le Case just up the street from Piazza S. Silvestro, is so delightful that it is difficult to imagine how Masson came to leave this important church out of her itinerary. Although some of the places touched on by Fort in Chapter 11 did not exist in Massons day, there can be no excuse for her not mentioning the National Gallery of Modern Art as she rushes her readers from the Etruscan Museum in Villa Giulia through Villa Borghese and on to Galleria Borghese, before it closes.

While Masson always seems to be in a hurry, Fort takes the readers at a much more leisurely pace, lingering over details that Masson overlooks. In the first chapter on the Capitol he includes a description of the Dying Gaul which she did not have. He ruminates over a Roman bust, the bronze of Lucius Junius Brutus (Bruto Capitolino) as typifying his mental image of what the Romans were like. He takes his readers through the Capitols Pinacoteca, which he tells them is a very mixed cake, but holds many plums. However mixed it may be it is a picture gallery that should not be missed, but Masson gave it no mention. In the church of S. Clemente he examines in detail the magnificent mosaics of the Triumph of the Cross behind the altar while Masson, who refers to them as the culminating triumph of the whole church, then sweeps on to the frescoes in the chapel of St Catherine of Alexandria. On other itineraries Fort stops in the Villa Farnesina to ponder the detail in the garlands on the ceiling of the Loggia of Psyche, and has time to search out the rooms of an 18-year-old Polish Jesuit saint behind the sacristy in S. Andrea al Quirinale. These are all things Masson missed. Forts addition of S. Susanna and St Pauls within-the-Walls, strange omissions in the original text, will please American readers who worship at these national churches.

Nowhere is the difference between the two editions more noticeable than in the opening to the chapter on the Borgo and St Peters. While Masson always has the pressed and hurried tourist in mind, Fort instead writes: For our purposes however, we shall assume there is not too much hurry....

Fort often manages to tone down that edge of frustration that creeps into the Masson original. Perhaps this is because the city today is better organised than it was in 1965, when Masson wrote her original text. Certainly there are many more and better-managed museums than in her day. However Fort readily admits that doing his research was not always easy. In Palazzo Barberini, for example, he says that nothing stays in the same place long enough to be certain that it will still be there on a second visit. Many sections of the Vatican Museums are now closed. In Palazzo Venezia parts of the permanent collection are never on view to the public. However in many other places, says Fort, he was given a warm welcome, and experts still refer to Massons Italian edition (there was only one, published in the early 1980s) with esteem and affection.

Clearly Fort enjoyed his year following in the steps of Masson, and probably if Masson were alive today she would applaud the more detailed, more leisurely and more baroque dimension that Fort has added to her work. Readers who have the time to compare the original and this revised version will have an invaluable insight into how much the city has changed in 40 years.

The Companion Guide to Rome by Georgina Masson, published by Boydell and Brewer Ltd, is on sale in or can be ordered by all English-language and international bookshops in Italy. The revised edition is both smaller and lighter than the original paperback. Publishers price 16.99 / $29.95.

Picture: John Fort and his wife Mary outside their apartment in Palazzo Doria Pamphilj.