Whilst this is not of immediate use, I can recommend a 3 volume book set called "Revised Manual of Gear Design" to anyone seriously into the subject. They are spiral bound volumes from the 1980's and although some of the work is a little dated (like maths tables) and in Imperial, there is still an awful lot of information - calculations and sketches - much more than the Mischke/Shigley books. One volume is dedicated to helical and spiral gears alone. The set is still available, I bought my last year from a UK bookshop new, and they cost 45UKP which I didnt think was bad. The books are written by E & EK Buckingham of Buckingham Associates Inc Gear Consultants, Parker Hill Road, Springfield, Vt 05156.