In writing my present book I have availed myself freely of the three works above mentioned. At the same time I have incorporated much fresh material; and I am glad to take this opportunity of stating, that, with the single exception of the Escorial, I have personally examined and measured every building which I have had occasion to describe; and many of the illustrations are from my own sketches. I call my book an _Essay_, because I wish to indicate that it is only an attempt to deal, in a summary fashion, with an extremely wide and interesting subject--a subject, too, which might easily be subdivided into separate heads each capable of more elaborate treatment. For instance, with regard to libraries in Religious Houses, I hope to see a book written, dealing not merely with the way in which the books were cared for, but with the subjects most generally studied, as indicated to us by the catalogues which have survived. A research such as I have had to undertake has naturally involved the co-operation of numerous librarians and others both in England and on the Continent. From all these officials I have experienced unfailing courtesy and kindness, and I beg them to accept this collective expression of my gratitude. To some, however, I am under such particular obligations, that I wish to mention them by name. In the first place I have to thank my friends Dr Jackson of Trinity College, Dr Sandys of S. John's College, Dr James of King's College, and F. J. H. Jenkinson, M.A., University Librarian, for their kind help in reading proofs and making suggestions. Dr Sandys devoted much time to the revision of the first chapter. As my work deals largely with monastic institutions it is almost needless to say that I have consulted and received efficient help from my old friend W. H. St John Hope, M.A., Assistant Secretary to the Society of Antiquaries. My researches in Rome were made easy to me by the unfailing kindness and ready help accorded on every occasion by Father C. J. Ehrle, S.J., Prefect of the Vatican Library. My best thanks are also due to Signor Rodolfo Lanciani, to Professor Petersen of the German Archeological Institute, Rome, and to Signor Guido Biagi of the Biblioteca Laurenziana, Florence. At Milan Monsignor Ceriani of the Ambrosian Library was so kind as to have the library photographed for my use. The courteous officials who administer the great libraries of Paris with so much ability, have assisted me in all my researches. I wish specially to thank in this place M. Leopold Delisle and M. Leon Dorez of the Bibliotheque Nationale; M. A. Franklin of the Bibliotheque Mazarine; M. H. Martin of the Bibliotheque de l'Arsenal; and M. A. Perate, Sous-Conservateur du Chateau de Versailles.