Mr. Willeford's book is very good for the how to. I hope you visit his webpage. Simplicity has a line of Steampunk costume. No comment. Many historical patterns can be made into Steampunk. Laughing Moon Mercentile, Truly Victorian, and Folkwear come to mind. I'd also suggest looking a actual clothing from the Victorian Era (at least to start with). Peterson's Magazine, Goodey's Magazine, etc. Fashion
by the Kyoto Costume Institute has some great examples. Masterpieces in Women's Costume
by Aline Berstein is another great one. The author looks at museum collections and it has patterns for some of the dresses. Finally, search for Steampunk costumes on this site. There are several excellent ones.
Edit: Harper's. I forgot Harper's magazine. Lots of late Victorian stuff there.