The term "reserves" was used a great deal during the last year of the war. It appears that county reserves was another name for first class militia, men not obliged to render Confederate military service due to age, occupation or disability who were expected to serve in the state militia. Any number of these commands were organized in Alabama (at least on paper) during the summer of 1864.

Randolph County Reserves who signed Federal paroles after the end of the war are listed in Alabama Confederate CSRs (reel 499), even though they weren't Confederate troops. However, since you cite "Mounted Men Reserves", I'd guess the publication you mentioned had a copy of one of the Randolph County Reserve rolls from the ADAH. Here's a list of what they have in Montgomery for Randolph County:

Breed's Company
Davis' Guards
Faulkner's Company
Faulkner's Cavalry Battalion
Ford's Company
Guinn's Company
Hunter's Company
Louina Guards
McClendon's Company
Randolph Beauregards
Randolph Blues
Randolph Mountaineers
Self's Company
Shephard's Cavalry Company
Wedowee Volunteers
Wesobulga Pioneers

About eight of these appear to be wartime militia or reserve companies; no telling which ones include muster rolls.

Sometime in the next three to four months I may be able to visit Montgomery again; I'll have to check with the travel agent (my wife) to see what's available. There's a lot of good genealogical material here which doesn't show up in actual Confederate service records. For instance, the Marshall County file includes a very fragile roll which provides detailed information about men from each beat who were NOT eligible for military service.