One reference book that might interest many of my readers is a Edward H. Landon's, "A Manual of Councils of the Holy Catholic Church." Published (as a new and revised edition) in 1909 in two volumes, it can be found on Archive.org. The arrangement of the volumes is a little counter-intuitive. It is arranged in alphabetical order by place name - not the order one might expect from a historical work, but nevertheless a useful order for a reference work. There are some useful appendices in Volume 2 for helping the reader to correlate the "modern" name of the councils with the Latin names and the actual places where the councils were held. The work is, of course, not limited to ecumenical councils. This may serve as a useful supplement to Hefele's great conciliar work.
Volume 1: A-N
Volume 2: O-Z plus Appendices
There are also older editions, but I am unable to find them on-line at this time.