This slim volume is an easy read with a lot of insightful commentary by a well-respected writer. I've never read any of Gardner's novels, but I may have to try one just to see how what he said about the writing process played out in practice. The book is a mix of "How to write" fundamentals that go deeper than just "Don't overuse adverbs" and personal reflections on how the writing process works for him. It's aimed quite explicitly at those who really want to make a career as a novelist, not just those who think they want to write a novel, and it's also heavily biased towards literary fiction; he tries to acknowledge sci-fi/fantasy and mysteries and other genres without sounding dismissive, but somewhat reluctantly.
Some of the most useful bits are when Gardner analyzes snippets from authors like Melville and Hemingway and himself, and shows why
a particular piece of writing does or does not show the author's best work, and how you can see even the masters developing their craft over time.
Besides the writing process, he also talks about the writing career, including what you can do with an MFA besides be a novelist (answer: teach), day jobs that are good for writers (there's a bit of naivete in his suggestion that any would-be writer can go get a job like fire watcher that allows lots of unsupervised writing time), and writing workshops. Some of the nuts and bolts stuff is dated now: Gardner died in 1982, so there is no mention of word processors or the internet, both of which have obviously had a major impact on the writing profession. But most of it is still relevant, and will always be relevant as long as there are novelists.