There are a few books and courses online that are free and really helpful when it comes to learning some of the things that will make you a better SEO. Knowledge can make a difference, and having an idea of how search engines work can possibly give you a competitive advantage over others who haven’t had a chance to learn about such resources. I’ve come across a few books that are online and free, and worth spending time with, and thought it might not be a bad idea to share them.
The first two volumes I found are ones that focus on one of the important ways that search engines understand the content of web pages, rating them based upon information retrieval scores. Having an idea of how a search engine might rank a page, based on more than just something of an understanding of how PageRank works can be really helpful.
Introduction to Information Retrieval by Christopher D. Manning, Prabhakar Raghavan and Hinrich Schütze