Given the significance, range and diversity of Rome's many churches (almost 1,000 in total), there are, surprisingly, few books in English which are devoted to the subject.
One of the few is June Hager's 'Pilgrimage', which was published on the eve of the 2000 Jubilee. In celebration of the event, many of the city's churches were given a well-deserved makeover and the accumulated grime of more than a century was removed. (Unfortunately, most of the photographs in the book seem to have been taken before this took place).
The book looks at the development of Christianity in Rome through forty of its churches. You will find all the usual suspects, as well as many less well-known examples, such as the delightful SS Nereo e Achilleo, which lies well beyond the typical tourist trail.
The book, unsurprisingly given its title, is aimed more at the pilgrim than the aficionado of architecture, but it is none the worse for that.