Essential Books for the serious study of Forster
You will notice that I have not included here any criticism, interpretation, or theory.  There is an imposing body of such works--reviews, articles, essays, books--which began accreting in the late 1930s and has grown quite large by the 2010s.  A few are definitely worth reading.  In the future I will undertake to make some specific recommendations.  One general bit of advice with respect to critical essays and books is to stick mainly to those written after Forster died and the fact of his homosexuality became generally known. My list below is based on the following priorities: Read as much as possible of Forster's own writing, not just his novels and short stories, but his published nonfiction (which includes biographies, essays, BBC broadcasts) as well as posthumously available letters, diaries, and journals. Know as much as you can about his personal history and experience. How else can you understand why and how he wrote what he did? Be aware of significant newly available publications and information.
Biography Furbank, P. N.  E. M. Forster: A Life.  2 vols.  London: Secker & Warburg, 1977-78. Furbank was a friend from the early 1950s until Forster's death and had been designated by him to write the inevitable biography (this overturned the earlier choice of William Plomer).  Furbank was the first to publish results of a meticulous reading of the letters and manuscripts left to King's College, Cambridge.  This book has been reprinted many times in a single volume in England and the United States.  As of 2012 it is still the most thorough biography. Beauman, Nicola.  Morgan: A Biography of E. M. Forster.  London, 1993. This book adds some new information, but it merely supplements Furbank, and gives short shrift to the last half of Forster's life. Moffat, Wendy.  A Great Unrecorded History: A New Life of E. M. Forster.  New York , 2010. This book gives weight to all periods of Forster's life and adds new information, insight, and emphasis, but does not supplant Furbank as the essential biography.  The title is a misnomer.  It is (mis)taken from a letter of Forster to his confidante Florence Barger (quoted on p. 162 of Moffat's book): ". . . I see beyond my own happiness and intimacy, occasional glimpses of the happiness of 1000s of others whose names I shall never hear, and I know that there is a great unrecorded history."  The significant fact about Forster's gay life is that it was recorded.  He wrote about it in letters and diaries and made a great point of preserving it and seeing that it would be made known.  So it is inapt to call a biography of him "A Great Unrecorded History."   Forster was applying the phrase to the truly unrecorded histories of the multitude of gay men who did not or could not document their lives.  Furthermore, Furbank's biography of Forster was published 30 years previously, using as an epigraph on the title page the following statement which Forster had written to his friend T. E. Lawrence ("of Arabia"): "But when I die and they write my life they can say everything."  And that is pretty much what Furbank did. King, Francis.  E. M. Forster and His World.  New York: Scribner's, 1978. This book offers the most extensive published photographic record of Forster in one book.  King's text offers some original insights by a fellow homosexual, but it is a rather catty narrative by someone who is not known to have been a friend of Forster's. Haag, Michael.  Alexandria: City of Memory.  New Haven: Yale University Press, 2004. Reproduces many wonderful photos of Alexandria from around the time Forster was there; also reproduces some documents and photographs from the King's College trove and traces Forster's Alexandrian adventure through his letters and journals from the period.  Also has similarly rich sections on Cavafy and Durrell.
Letters Selected Letters of E. M. Forster.  2 vols.  Ed. Mary Lago and P. N. Furbank.  Cambridge: Harvard UP, 1983-85. Calendar of the Letters of E. M. Forster.  Compiled by Mary Lago.   London: Mansell Publishing, 1985. Forster corresponded prodigiously throughout his life, writing letters, notes, and cards almost entirely by hand.  The Selected Letters prints a mere 446 of the almost 12,000 items listed in Mary Lago's amazing Calendar, which admirably lists (by recipient) all known letters and their whereabouts as of 1985.   Each entry includes the date and place of writing (if indicated in the letter) and the opening phrase.  New Forster correspondence continues to come to light, and a supplement or new edition is needed.
Bibliography Kirkpatrick, B. J.  A Bibliography of E. M. Forster.  2nd ed.  Oxford: Clarendon, 1985. An extraordinary and almost impeccable record of Forster's publications and manuscripts. A resource to be mined incessantly. Forster worked with Kirkpatrick on the first edition and contributed a foreword.  "I am surprised and glad to discover from this bibliography that I have written so much." The 2nd edition was much expanded and is the one you should consult.  Again, a supplement or new edition is needed.
New or Notable Editions of works by Forster Alexandria: A History and a Guide and Pharos and Pharillon.  Edited by Miriam Allott.  Abinger Edition 16.  London: Andre Deutsch, 2004. This is the last volume in the scholarly Abinger Edition, which has been 30 years in the making.  Ample introductory material and notes, though all and all a rather perfunctory exercise.  Its primary value is that it reprints Forster's essay "Egypt" from the 1920 Labour Party pamphlet and, perhaps most importantly of all, it includes Forster's intimate journal in the form of a letter recording the history of his relationship with Mohammed el Adl.  No illustrations.  The BBC Talks of E. M. Forster: A Selected Edition.  Edited by Mary Lago, Linda K. Hughes and Elizabeth MacLeod Walls.  Foreword by P. N. Furbank. Columbia and London: University of Missouri Press, 2008.  ISBN 9780826218001. It is good to have this satisfying collection of broadcast texts, which reveals a less known--but appealing--side to Forster.  One longs to have the surviving sound or video recordings of this acute yet affable wise man of letters made available on CDs or DVDs. The Feminine Note in Literature.   Edited with an Introduction by George Piggford.  The Bloomsbury Heritage Series 28.  London: Cecil Woolf, 2001. A scholarly and meticulous presentation of the text of a talk given twice by Forster in 1910.  Heretofore this thought-provoking essay could only be seen in manuscript at King's College.  The editor speculates, in his informed and useful introduction, that the essay, which Virginia Stephen (later Woolf) heard in December 1910, may have been among the influences that led to her now famous statement: "On or about December 1910 human character changed."  Since Forster's writing has been both praised and blamed for its "feminine" traits, it is illuminating to read his reflections on the characteristics of novels written by women.