Exceptionally rare with no copies available. 

H. Dunning. The Hour of Prayer, or A Year at the Union Prayer Meeting. A Discourse wherein is given some account of the Union Prayer Meeting, held in the Presbyterian Church, (Green & German Sts.) Baltimore, Maryland. Baltimore: Hanzsche & Co., 1859. 15pp.

Good, solid original wraps with some toning and fading to extremities. Solid and clean though. Not in Roberts. Very scarce. 

The "Union Prayer Meeting" was the name given to the Prayer Revival in some quarters.  This particular message is dedicated to telling the history of the revival as it happened in Baltimore. Very rare, first-hand history.