A basic science text, a little out of date now, on how genetics work in humans - though admittedly, the gene explanations start (as they so often do) with Mendel and his pea plants before moving on to blood types and so on. Very easy to understand, if not particularly thrilling.

I enjoy reading Asimov's non-fiction more than his fiction I think. At least his non-fiction on science - having a sci-comm background myself, it's always interesting to see how Asimov interprets for lay-people, and to compare his science writing to that of other science communicators. Which sounds very boring I know, but looking at the different ways people talk about science helps me to talk about it more clearly myself.