Darby was one of the founders of the Plymouth Brethren movement.Two of the most notable brethren are F. F. Bruce (1910-1990) and Harry A. Ironside (1876-1951). I have two books in my library on the Plymouth Brethren: My People: The History of Those Christians Sometimes Called Plymouth Brethren (Harold Shaw, 1995), by Robert H. Baylis, and The Plymouth Brethren (Oxford, 2018), by Massimo Introvigne. The former book contains 336 pages, including over 100 illustrations and photographs. The latter book is much smaller (141 pgs.), but is the latest research on the subject.
I am embarrassed to say that I have not read and do not own H. A. Ironside’s A Historical Sketch of the Brethren Movement: An Account of its Inception, Progress, Principles and Failures and its Lessons for Present Day Believers (Zondervan, 1942). I would read it before the two other books.