Gwilym Lleyn was still a young man when he began to interest himself in the collection and listing of books in Welsh and books relating to Wales. His work as a circuit minister gave a searcher such as he was a wonderful opportunity to examine and collect books. The first fruits of his researches appeared in Y Traethodydd for 1852-3, under the title of ‘Llyfryddiaeth y Cymry’ (‘Bibliography of the Welsh’). He proposed to issue the work, when completed, as a fifteen-shilling volume. The work was not published, however, until 1869, four years after his death, when it was prepared for the press and edited by Daniel Silvan Evans (q.v.) and printed by John Pryse (q.v.) of Llanidloes, under the title of Cambrian Bibliography: containing an account of the books printed in the Welsh language, or relating to Wales, from the year 1546 to the end of the eighteenth century; with biographical notices. There was also a Welsh title-page, Llyfryddiaeth y Cymry …, and it is under this Welsh title that bibliographers and historians of literature in Wales and elsewhere know the work and refer to it. This was at the time a notable achievement, and, although it has its imperfections and its errors of fact, the book continues to be a most useful work of reference; when it is realised that there were in the time of the compiler no Welsh libraries at Cardiff, Aberystwyth, or Bangor, the achievement of the author must be regarded as remarkable.

Rowlands edited Yr Eurgrawn Wesleaidd from 1842 to 1845, and again from 1852 to 1856; he also contributed to periodicals such as Golud yr Oes and Y Brython. His other published works were concerned with religion and denominational controversy. He wrote a series of biographical articles on famous Welshmen; after his death the manuscript of the work was acquired by Isaac Foulkes (q.v.) who incorporated some of the material in his Geiriadur Bywgraffiadol o Enwogion Cymru (Liverpool, 1870), and referred to it there as the ‘Lleyn MSS.’ A memoir of Rowlands, by his son-in-law, R. Morgan, appeared in the twelve issues of Yr Eurgrawn Wesleaidd for 1868; see also D. Silvan Evans's preface to Llyfryddiaeth y Cymry.