Books on this subject are few and the older ones are hard to find. The best introduction is E Clive Rouse, Medieval Wall Paintings, Shire Publications, 1991. Brief but expert treatment of the subject, with good pictures, a Gazetteer and a very useful Bibliography. Some cathedrals and larger churches may have this book on sale inside.

Also useful are:
A Caiger-Smith, English Medieval Mural Paintings, OUP, 1963. Thorough and scholarly, with good pictures.
H Munro Cautley, Suffolk Churches and their treasures, 5th edition with Supplement, Boydell Press, 1982. Exhaustive coverage of Suffolk Churches.
John Harvey, Gothic England:A Survey of National Culture, 2nd edn. Batsford, 1945. Contentious, and (some say) eccentric at times, but stimulating.
MR James, Suffolk & Norfolk, 1987 edn. Alastair Press, Suffolk. Subtitled ‘A Perambulation of the two counties with notices of their history and their ancient buildings’. Still superb, still indispensable.
Frank Kendon, Mural Paintings in English Churches during the Middle Ages, John Lane/Bodley Head, 1923. A bit dated, but particularly useful for its summarising of material from:-
CE Keyser, List of Buildings having Mural Decoration, London (HMSO), 1883 (‘Keyser’s List’). Now out of date, but an updated version is in preparation under the supervision of the Courtauld Institute.
EW Tristram, English Medieval Wall Painting : The 12th Century, London, 1944.
EW Tristram, English Medieval Wall Painting : The 13th Century, London, 1950.
EW Tristram, English Medieval Wall Painting : The 14th Century, London, 1954.
EW Tristram & T Borenius, English Medieval Wall Painting, London, 1929.
Professor Tristram’s monumental series of volumes is comprehensive, magisterial, and, the odd eccentric iconographical reading apart, reliable.
JC Wall Medieval Wall Paintings, London, n.d. (before 1925). Invaluable for Walls drawings, some of them of paintings now vanished.
PG Walwin, St.Christopher Today and Yesterday, Gloucester, 1968.
HC Whaite, St. Christopher in English Medieval Wallpainting, London, 1929. A volume devoted to the most frequently found of all subjects
Richard Marks, Image and Devotion in Late Medieval England, Sutton Publishing, 2004. Not specifically on wall paintings, but by far the best book in recent years on this image-related area of medieval culture. Comprehensive and illuminating, with some superb photographs. NEW
Daniel V Thompson, The Materials & Technique of Medieval Painting, Dover Publications, New York, 1936. Also not specifically on wall paintings, but still the standard work on materials/technique. Recently reprinted and widely available NEW