Catholic church and Christian state by Vatican archivist
Hergenröther's Catholic church and Christian state; a series of essays on the relation of the church to the civil power
Vol. 1, Essay V, part I (pp. 205 ff.) was a very good concise overview of the Syllabus
's doctrinal status.
Vol. 2, Essay XVII, "The Church & Liberty of Conscience (pp. 347-384) on Syllabus
15-18, 77-79 seems the most interesting.