My suggestion, if you have the time, is to leave a few comments on their blog first over a short period of time so that you can start to build that relationship with them; that will do more to get your email opened than an individual subject line. They have to be good comments though, that are relevant to the post you are writing on, and not just spam. If somebody recognizes your name because you've been a good commenter on their blog, then they'll be more willing to reciprocate your attention by giving you some of theirs.
For the actual subject lines to use, keep a document that lists the lines that you use and which ones you get a response to. Testing is the best way to see what works.
I haven't done a lot of emailing folks with whom I don't have a pre-existing relationship, but here are some lines that could be worth trying (in no particular order):
- Do you have a book?
- Are you published yet?
- Do you have a book in ______?
(fill in w/a local bookstore or industry specific store in their area, use google to see what's in their city – if they don't list where they are from on an about or contact page, you can lookup their whois information using any domain registrar to see if that's publicly listed or not in their domain.)
- Want ______ to be a best-seller on Amazon?
(come up w/a decent book title based on their blog – even if they don't hire you, you'll have provided a little bit of value.)
My basic rule of thumb: Avoid spammy behavior, don't use all caps, avoid exclamation points, ask questions, be relevant, and don't be deceptive.