People sign up to your list because they are interested in content you provide. No one will mind, and it will certainly improve your sales once you do launch.

I did this with my book. 0 unsubscribes.

It wont seem spammy if it is well executed. Just make it easy to unsubscribe from the tips without unsubscribing from the book notification. Also, make it clear in the first email that the recipients only signed up to get notified about the book but that you thought it would have been useful to send periodic tips. Also be sure that the tips have are posted on the web, so they can be shared and viewed without having to reference an email.

Much like others have said, when people sign up for content giving them content isn't spammy. Do include an unsubscribe link. Also include a subscribe link, include content worth forwarding, and sell more books.

Edited to add: Might want to have a "just tell me when the book is ready, no more o' these" just in case. But really, if people aren't interested in content similar to what's in the book, why are they buying the book?

Actually it's a pretty good idea, maybe even better than the book itself. I'd be glad to receive vim tips everyday in my e-mail.

You could also take the approach the Django Book originally did. Posting the book online while you write, with editing help from the community and a print version available once complete. I would definitely buy the print version as well as use the web reference.

I agree to the daily or maybe have an option for a weekly thing, but I am not sure how this will match up with the model/book you have planned out so far.

I think it's a great idea, especially if the tips are concise and easy to absorb.

send the emails with an unsubscribe link. you'll be fine.

Yes please.

I would not view that as spammy at all.