Soon after settling at Cambridge, Maitland perceived that his vast design of interpreting English law stood in need of cooperative effort. He consequently succeeded in 1887 in founding the Selden Society, 'to encourage the study and advance the knowledge of the history of English law' by publishing needful material, with headquarters in the Inns of Court in London, and under the direction of the legal authorities. In the twenty years intervening between its foundation and Maitland's death the society issued twenty-one volumes on the history of different branches of the law, edited either by himself or by editors selected and supervised by himself. In 1887, too, the year of the Society's formation, he published his first important work, 'Bracton's Note-book' (3 vols.). It was an edition of a British Museum MS. which he put forward as the actual materials collected by Bracton [q. v.] for his great treatise 'De Legibus et consuetudinibus Angliaj,' temp. Henry III, one of the best sources of English history and law in the period immediately preceding Edward I. In 1887-8 he delivered a course of lectures at Cambridge on 'The constitutional history of England' from the death of Edward I to his own. time (published after his death). In 1889 he published two moat important contributions to periodicals: 'The Materials for English Legal History' in the 'Political Science Quarterly,' being a thorough analysis and classification of all known available materials for each period from Ethelbert to Henry VIII, and 'The History of the Register of Original Writs' in the 'Harvard Law Review,' an admirable illustration of the proper method of dealing with one of the most abstruse branches of his materials—the development of the forms of action at common law. Meanwhile Maitland was actively engaged on his 'History of English Law before the Time of Edward I,' a magnum opus which he planned in consultation and co-operation with Sir Frederick Pollock. The work, published in 1895 (2 vols.), bears the names both of Sir Frederick Pollock and Maitland on the title-page, but it was substantially carried out by Maitland. It was at once universally adopted as an authoritative textbook on this period and a model for other periods. In the same year (1895) he was made literary director of the Selden Society.
Maitland next turned his attention to a different theme, the action and reaction of Roman civil law, whether ancient or mediæval, upon English law. 'In 1895 he traced the sources of the influences of Roman law upon English law in the thirteenth century, in a volume, 'Bracton and Azo,' issued by the Selden Society (viii.).
Carrying his study of the topic down to the sixteenth century, he confuted, to the annoyance of Anglican apologists. the partisan theory that there was in England before the Reformation a system of Anglican canon law independent of the Roman canon law. After several essays in periodicals through 1896-7 (see Collected Papers) he published in 1898 his 'Roman Canon Law in the Church of England,' finally proving that the pre-Reformation canon law enforced in England was purely Roman. His judgment was accepted, even by Stubbs, who was in part responsible for the other theory. Free from all theological bias, Maitland regarded the Reformation as a national movement of statesmen, using royal necessities and reformers' enthusiasm to deliver England from the actual oppression of Papal canon law and the prospective infliction of the mediæval civil law. Further researches into the legal effect of the Reformation led to dissertations on 'The Corporation Sole, the Crown as Corporation,' 'The General Law of Corporations,' and 'Trust and Corporation'—a study of the growth of 'trusts' as an elusive but effective substitute for the strict legal corporation. Maitland's scholarly impartiality received conspicuous recognition. On Lord Acton's invitation he wrote on the 'Anglican Settlement and Scottish Reformation' in the 'Cambridge Modern History' (1903).
Convinced of the inadequacy of the printed texts of the Year Books in old legal Anglo-French, Maitland persuaded the Selden Society to undertake a new edition, selecting the period of Edward II, with a careful collation of all MSS., translation, illustrations from the plea rolls, and introductory essays. With the assistance of Mr. G. J. Turner, Maitland produced the first three volumes in 1903-4-5. The fourth volume was completed after Maitland's death by Mr. Turner in 1907. For his own use Maitland compiled a grammar of the old law-French, and published it in the introduction to the first volume.
At the same time Maitland, apart from his historical studies, advocated many plans of legal reform, such as the simplification of English law by the abolition of the separate law of real property 'founded on worn-out theories and obsolescent ideas' ('The Law of Real Property,' 1879; 'Survey of a Century,' 1901, in Coll. Papers), and
