Former Ph.D. student needs introductory biology or biochemistry textbook

Hi, I am in my 40's and on Social Security and SSI. I already have my B.A. summa cum laude with Highest Distinction in linguistics from a top 25 school, and also was a former Ph.D. student in the same field at an Ivy League institution. I have only a high school science and math background, but I always did well in those areas. Unfortunately, graduate linguistics didn't work out for personal reasons (nothing to do with academics). Perhaps that is good, as here are no jobs in linguistics anyway (I have watched many of my former classmates struggle finding jobs in this field, even with a Ph.D.); and I learned I really don't have an interest in that field. I want to get off Social Security, finish my education, and get a job. I have decided to switch and get my Ph.D. in a more marketable area, a STEM (science, technology, engineering, math) field. I want to do cancer and/or obesity research since they have affected my family tremendously, and I have found I have a real passion for those areas. So, would biochemistry be right for me? Or biology? Or chemistry? A Ph.D. is one of these areas is a must for me. I thrive in academe.

Since I already have a B.A. from the early 90's and consider myself to be pretty intelligent and diligent (not trying to brag here), and since I don't have much money, I just plan to wing it, and give myself the equivalent of a "home-schooled" undergraduate education in one of the above science fields in just a few years. I will take some community college courses (free for low income people in California) for academic references, and beg my way into labs or internships if I need do. The sooner I get this done, the sooner I can apply for graduate school.

So, I need to get some introductory books to study on my own. Does anybody have any suggestions, or other advice for me?

Thanks!