The best source for architectural information on English churches can arguably be found in the series of books written by Nikolaus Pevsner (The Buildings of England series). The series is being revised, and expanded to cover other parts of the UK and Ireland. Not cheap, they can often be found in second-hand bookshops.

Free e-books.

Simon Jenkins' "England's Thousand Best Churches", Penguin, ISBN 0-140-29795-2 is highly recommended.

Another fascinating guidebook is Brian L. Harris's "Harris's Guide to Churches and Cathedrals", published by Ted Smart in 2006. ISBN 0-091-91614-3. It covers England only.

Other useful sources are the "Collins Guide to Parish Churches of England and Wales," edited by John Betjeman, ISBN 0 00 216166 4, and Paul Johnson's "Cathedrals of England, Scotland and Wales", ISBN 0 297 83013 9.

An excellent book for the dedicated church visitor - "How To Read A Church," which describes itself as "a guide to images, symbols and meanings in churches and cathedrals," by Richard Taylor; ISBN 184413053-3.

I've recently acquired a rather battered copy of an old book, "Anglican Church Architecture", by James Barr, published in 1866. I hope to transcribe it at some point and add it to the website.

Rhonda Houston has recommended Stephen Friar's "A Companion to the English Parish Church", which, despite its title, also covers Ireland and Scotland; ISBN 1858337380.
 

Tom Wesley recommends "Churches in the Landscape", by Richard Morris, 1997 Weidenfeld & Nicholson. ISBN 0753801175. He describes it as "a fascinating book on why churches are where they are".

George Weston recommends the four volume series "Non-Conformist Chapels & Meeting-Houses". All by Christopher Stell, they cover South-West England (Vol. 1, ISBN 0-11-300036-7), the North of England (Vol. 2, ISBN 0-11-300041-3), Central England (Vol. 3, ISBN 0-11-701181-9), and Eastern England (Vol. 4, ISBN 1-873592-50-7). Vols 1-3 are published by the Royal Commission on Historical Monuments, and Vol. 4 by English Heritage.

This one will require searching second-hand bookshops or a web search, as it is out of print. Ian Thirlwell recommends the softback booklet "Churches, Chapels and Places of Worship on Portsea Island", by John Offord, published in 1989.

Kevin Gordon recommends "A Little History of the English Country Church" by Roy Strong. (2007 Jonathan Cape 264pp ISBN: 9780224075220).
 

If anyone can recommend other books on the churches of these islands, I would be pleased to include them here.