I recommend any Schaums Outline book on the matter. I've read several of them and, although there are books that are better and more specific in many instances, they follow a very easy to grasp format that is common to all other Schaums Outline books.
Therefore, they are a great start for self-studying any subject. The main good point is that, as they are not too dense, the chances you'll leave the book half-through, and stop studying in general, decrease a lot. They will increase your curiosity and understanding, so after reading them you should be ready to read more dense books later on.
I want to clarify that they are not equivalent to "for dummies" books at all. Although they are not dense, they are written assuming you are not dumb and under the principle of being able to make you pass your tests without attending classes. So they are serious books made for practical purposes; no pretence whatsoever.
Particularly on Biology, there are several books:
Schaum's Outline of Biochemistry
Schaum's Outline of Biology
Schaum's Outline of Microbiology
Schaums Outline Of Genetics
Etc. You can learn about any science with them, really.
I'll leave a link with more details on the one in Biology:
They have books on many other sciences. They are designed for self-study.