You can go to google at use advance search strings to find a list of sites related to investing and put the domains into note book. Try to get as many as possible. Then head over to alexa and see how many people go to each site. If the amount of traffic is descent you can then see how they advertise. Ask them if you can advertise on there site and try to broker the best rate you can. You might want to start charging more for your book since it will help the presumption of value. If your book is good quality and actually does what you claim it does then your book is much more valuable then 4 and some change. make sure to test your landing page and see how well it's converting. Tweek it here and there to see if you can't improve this rate. If you have a conversion rate above 1% then you have a winner that can make you much more money then your teaching your costumers to make.