An incredible book from an incredible writer. In one sentence de Camp provides an excellent means to know if you need to learn more about English or not. To wit:
"If you see nothing wrong with such sentences as 'He stared like he had seen a ghost,' or 'An individual can return to any period of his entire life providing his passage is not blocked by engrams,' you do not know enough about writing English to tackle fiction."
My copy is the 1975 edition, and it is well-worn from frequent reading.
The book starts with an (at that time) fairly exhaustive history of imaginative fiction, moves on to discuss the modern genres, and then editors and publishers, and writers and readers.
Their advice on preparing for a science fiction career, on getting ideas, on plotting, on writing, and on selling is, on the whole, still valid. If you ignore the bits about copyright (since this book pre-dates the 1976 Act and the 1998 Sonny Bono Act) and that the business section pre-dates the personal computer, you can still garner lots of useful advice from the entire book.