The elders need to continually be developing leaders. Bob Deffinbaugh, in this five-part series, challenges elders to be on the lookout for potential leaders and to take steps in developing them.
The Training of the Twelve by A.B. Bruce, Kregel Publications, 1971. This classic, written originally in 1894, is on the top ten reads for both, Dr. Howard Hendricks (legendary teacher of Dallas Theological Seminary) and George Verwer, founder of Operation Mobilization. This book examines the ministry of Jesus to his twelve disciples. Short chapters, but deep in spiritual insight.
18 Words for New Elders. Excellent article for new (and longstanding) elders, summary advice from a variety of experienced elders.