Dawkins' books, especially the earlier ones (e.g. Blind Watchmaker), are quite a good and gentle introduction.

Yes, you are right that life originally started as asexual reproduction and sexual reproduction appeared later; my answer was more about the likelihood of a species that already reproduces sexually moving towards asexual reproduction.

Regarding the question of why the asexual->sexual shift happened in the first place, the wikipedia article on asexual reproduction has this to say:

Current hypotheses [2] suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments.