Ive got this one
https://www.amazon.com/Rock-Climbing-Virginia-Maryland-State/dp/0762784342/ref=dp_ob_title_bk
But I think it was cheaper at REI, maybe 35$.
If you are cost conscious and want to buy a guidebook that covers a lot of areas but has isn't quite as good for Seneca as the Barnes guide, this works: http://www.amazon.com/gp/product/0762784342 Unless you are mid-week, you will not be alone, so be prepared with a backup plan if your route of choice is taken. Much time can be saved by having pre-planned some alternates near your first choices.
Don't be afraid to ask for specific beta on routes you want to do here or elsewhere - I think there's a relatively active "Climbing Seneca Rocks" facebook page.
I'd get some cams or ask the local guides for recommendations on which routes go best with passive pro if you really want to do the 5.6 - 5.7 stuff first. Certainly most of the classics were put up with passive gear and big boots, and I think you'd have a good time on the classics (Ecstasy Jr 5.4, Old Mans, Old Ladies, Gunsight to South Peak, etc).
If I had to choose between a #3 cam and a helmet I'd bring the helmet any day - it's not a choss pile but there are lots of ledges and little rocks.
If you have the money and run short of partners, both guide services in town are great.
Relevant archived post: https://www.reddit.com/r/climbing/comments/1m14zr/first_trad_climb_at_seneca_rocks/