This classic book lays out, as you might expect, a technique for producing ideas. It’s very short and simple. In a nutshell, you must maintain a good supply of general knowledge, steep yourself in specialist knowledge about your problem, and then forget about it and go and do something else. The ideas will come as if by magic.
This book is almost 50 years old, but its technique is still good today. I’ve read these ideas in various other current books and on the web, so it seems that many people are discovering it even today. It does seem to make sense, and for me at least it works.