A free book/resource for learning genetics?
As you said, the question is pretty broad. Genetics is a big gigantic field and it is quite hard to know what you are exactly looking for. If you could refine into one of the subfields (molecular genetics, population genetics, phylogenetics, etc..) it would be much easier to give you better advice. As you talk about both biology and bioinformatics, it might be worth to specify whether you are looking for learning more in fundamental genetics or learning more in the methodologies to analyse genetic data.

Principles of Genetics

You may want in an introductory class in genetics. Here is a course on MIT Open CourseWare, but it may be too easy for you.

Bioinformatics

You may want to take a bioinformatics course. There are so many bioinformatics course online that there is a wikipedia page to list them (there).

Side skills

Programming

You may just want to learn to code in R, Python, Perl or Shell script (to cite few examples). You'll find tons of beginner tutorial online.

Statistics

Or you might want to learn more about statistics. KhanAcademy has a very good (but very introductory) statistics class.