This has been posted many times before, but probably deserves another look.

NARA site containing descriptive pamphlets for download:
1. Headquarters Records
2. State Records
- - Records of Superintendents of Education (AL, AR, DC, GA, LA, NC, TN, TX, VA)
- - Records of Assistant Commissioners (AL, AR, DC, GA, LA, LA-NO, MS, NC, SC, TN, TX, VA)
- - Field Office Records (AL, AR, DC, FL, GA, KY, LA, MD/DE, MS, MS-Pre-Bureau, MO, NC, SC, TN, TX,VA)

3. Marriage Records (Headquarters)
4. Office of the Adjutant General

The NARA site contains a link to each of the associated Descriptive Pamphlets (DPs) in .pdf format.

The link below is to the DP for Texas Field office records (for example). It describes the organization of the Field Offices, locations, and available records for each location.

http://www.archives.gov/research/microfilm/m1912.pdf

The NARA link provides an overview to all available records/resources: