I would like to share a good resource with you. As a return favor please let me know other pdf search engines (free only). PDF Manual Search Engine is a 'not at all bad' place where you can find pdf ebooks related to your subject of interest. I could find some good (as well as bad LOL) ebooks regarding search engine optimization. But as long as it is free it's okay.
Hope the same information from you too. Thanks.
You may check a 'bulk' list of ebooks related to seo here. But choose the right ones before you download - pdfstack.com/pdf/seo-rdf.html