Old books can be good, too. I like
- Opticks: Or, a Treatise of the Reflexions, Refractions, Inflexions and Colours of Light by Isaac Newton. Unlike the Principia it's written in English and full of the details of Newton's experiments with light, along with discussions of more general scientific matters.
- On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life by Charles Darwin is also very enjoyable. I read the 6th edition; this contains Darwin's comments on what other people have said about the previous editions. I really enjoyed reading Darwin's gradual development of his arguments and being very impressed by how convincing the book is, given the relatively sketchy knowledge of palaeontology, geology, and indeed biology when it was written.
Nigel (UK)