In 1856, he was appointed assistant prefect of discipline and sub-librarian in the College of the Immaculate Conception, Rue Vaugirard, Paris.
The Bibliothèque of Augustin and Aloys de Backer was then in course of publication, and Sommervogel, noting its occasional errors and omissions, made a systematic examination of the whole work.
Four years later, Augustin de Backer, seeing his list of addenda and errata, a manuscript of 800 pages, containing over 10,000 entries, obtained leave to make use of it.
Aloys de Backer in the revised edition of his Bibliothèque (1869–76) gave Sommervogel's name as co-author, due to the latter's significant contributions.
He determined to recast and enlarge their work and after five years issued the first volume of the first part (Brussels and Paris, 1890); by 1900 the ninth volume had appeared; the tenth, an index of the first nine, which comprised the bibliographic part of the Bibliothèque was unfinished at the time of his death but was later completed by Joseph Brucker, from which these details were drawn.