The ten volumes of Stewart Sifakis's Confederate answer to Dyer's Compendium have never been pronounced perfect, but until something better comes around they remain the best quick reference unit guides available.
Indeed, something better has come along in some instances. Improvements on the theme include Art Bergeron's sterling Guide to Louisiana Confederate Military Units and Jim McGhee's new release (just now) Guide to Missouri Confederate Units, 1861–1865. I haven't had a chance to read Ralph Wooster's Lone Star Regiments in Gray or Clark's Regiments for North Carolina.
The hardcover editions of the Sifakis set are out of print, with wildly varying prices on the secondary market, but Heritage Books, Inc. has reprinted them in paperback format, at a price range from $25-$30 -- providing a serviceable, if rather overpriced, fall back position for obtaining a complete set.
Volumes - Compendium of the Confederate Armies: