Standard text on Beethoven's sketches:
The Beethoven Sketchbooks / History, Reconstruction Inventory. Douglas Johnson (early period), Alan Tyson (middle period), and Robert Winter (late period). Edited by Johnson. Berkeley: Univ. of California, 1985. 611 pp.
Sketch catalog: "SV" numbers:
"Verzeichnis der Skizzen Beethovens" ("List of Beethoven's Sketches") in Beethoven-Jahrbuch 6 (Jahrgang 1965/68, 1969): 7-128. Prepared by Hans Schmidt.
Lists sketches by city, gives each a number and limited bibliographic information, and has an index by Opus, WoO, and Hess numbers, and a list of sketches for incomplete works. The list is unfortunately incomplete and inaccurate; some corrections may be made by consulting no. 4a.
For other major studies of Beethoven sketches, search in the Beethoven Gateway in the Category index (search term: sketch studies). Additional books and articles that discuss Beethoven's sketches may be identified by searching "compositional process" in the subject indexes.