This is an interesting idea. Do you already have professors creating quality textbooks using your service? I am just curious because creating a good textbook from scratch requires a lot of effort and time.
You should be reaching out to professors who are keen about the latest technologies because they are the ones that are most likely to use your service. Linkedin is a good place to start. Also, I found this list that you can start with - 100 Best Professors Who Blog.
When reaching out to them, try to ask for feedback (they will actually use your service in order to come up with one) instead of asking them to promote your website. Work on their suggestions, if any and always follow up once a feature that they requested for is introduced. If they like your website a lot, ask for contacts that they think would be interested in using your website. From there, you will start to get a list of individuals that you can get in touch with.