Read up on on-page SEO. Your "finance" category should be optimised for "finance ebooks" or "download finance ebooks" or similar. Likewise for every other page on the site.
Make the titles unique on every page, add a relevant H1 heading to every page (your site name should not be the H1). Add a paragraph of unique content to every page. Make the meta description / keywords unique on every page, or don't use them at all.
Many more on-page changes are required for this site, but I'm not able to list them all in a forum post - plenty of good online resources for this.
Ask yourself, does my site add value? Google is getting better at giving out bad rankings to sites without much unique content. Make sure your site isn't one of those sites.
Add value by having user or admin reviews of the downloads, a user-rating system, links to similar resources etc etc.
Decide what phrases you want to rank for - do this first. Then do the on-page SEO required for those phrases, then build links with those phrases as the link text.
If you are serious about learning SEO, I would shell out the $80 for SEO Book from
www.seobook.com (no affiliate link, honest) - you can get the info for free, but SEO book has all the info in one place and is free from BS often found in forums etc.