Pick the man-made non-living thing

The correct option is Books.

To understand why this is the correct answer, let us look at the differences between natural things and man-made things, as well as living and non-living things:
1. Natural vs. Man-made: Natural things exist in nature and are not created by humans. Examples include mountains, valleys, and rivers. Man-made things are created by human beings using various materials. Books are manufactured by humans using paper, ink, and binding materials.
2. Living vs. Non-living: Living things can grow, breathe, reproduce, and respond to stimuli. Non-living things do not possess these life processes. Books cannot breathe, grow, or reproduce, which makes them non-living.

Let us analyze the given options:
- Mountains: Natural and non-living.
- Valleys: Natural and non-living.
- Books: Man-made and non-living.
- Rivers: Natural and non-living.

Therefore, Books is the only man-made non-living thing among the choices.