[alecdibble]

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.