Niche Sites And Adsense Resources, Randy's post follow up

Randy made a post in another thread that made a lot of sense regarding creating niche sites as a way to create multiple streams of income. This is something that I have been interested in for a while. I do currently have several sites, but they are in related fields. I wouldn't consider them "niche" sites, because they are in rather competitive areas.

I like the idea of slowly building sites to help diversify and bring in more income, but I don't really know where to start, how adsense works, etc. I have a lot of questions, like how many people searching for a product is enough to make it worthwhile without being too many, making it too competitive? How can I come up with product/subject ideas?

Are there any resources, websites, e-books, etc that you can recommend that might have some answers/ be a good resource for someone who is interested in learning more about niche marketing?

Thanks!

Well i will not promote my business.
But i have 3 books which i bought from a some well known websites worth $19 , $21 and $10
I can provide you at a very cheap rate.
Honestly speaking.