Hi,

As Vadym wrote, I think that 4 emails over 30 days is a MUCH too little spread out, I'm sending three to four emails per week. I'm the IM niche I send most emails with content and goodies and the forth or the fifth is a salespitch. while in niches like yours with less competition, with small and responsive community I would try to create fresh content to target new subscribers, to place ads in non related sites as in small niches, people do not always know where to search for information and they will be happy to find leads to guide them.

Don't know a lot about that niche but after Googling it and thinking about, I would try to "think different", to send at least 12 per month while most of the emails will have good and helpful content, I think you can come up with a lot of racing related, parts related, news, security, related products content in the subject and although it's a good idea to JV with the "one and only ebook owner" I would try to find a good writer and to create some exclusive content, a new ebook which would help you to grow a targeted list and gain reputation.

Again as Vadym suggested "Don't worry about unsubscribers", You should concentrate in building a targeted and responsive list, but, I wouldn't use the "Self Development" as the main email campaign, I would warm the list with "the subject related content" while dripping the "Self Development" content throughout the campaign as a supplementary content.

Have Fun,
Ziki de Naim