This is the big one - if Sandman was Gaiman's breakout role, his work on American Gods is a big part of what made him who he is today. American Gods, however, was not Gaiman's first novel. Before it he wrote Neverwhere, Stardust, and co-wrote Good Omens with Terry Pratchett.
Going over these three novels first, each one has a great lesson to learn from it.
This piece is a great example of something that Gaiman has demonstrated again and again - working with others is super important, even for writers. The origin of the piece was a short story that Gaiman had wrote and, in his words, he "didn't know what happened next". So, he sent the piece on to Pratchett to take a look at and see if he could do something with it. Pratchett took a look and thought he had an idea. They ended up collaborating on it and co-wrote the book together, eventually touring.
There's a strange trend out there right now of authors decrying collaborating - saying that its not worth it, that most people are not good at collaborating, and that there is just too much ego going on in a process like this. Gaiman and Pratchett, however, made one of their greatest books as a result of this collaboration, and from my observation point also learned a lot from each about both writing and the process of marketing a book through this experience.
The creation of Neverwhere is a fascinating story and a great lesson for writers. First developed as a television series for BBC, and it has since become a novel (released while the show aired in 1996), a stage production, a radio drama, and other things. The television show was also originally co-created by Gaiman and another creator, though the novelization appears to be solely Gaiman. In interviews, Gaiman has alluded to the novel being a requirement as it cleared up some of the changes and, in his mind, the shortcomings that he felt existed with the television show. This demonstrates a commitment to vision of his work and a willingness to reexamine it again and again to ensure that it is right.
Not only does this teach important lessons about working with others and demonstrating that for complicated works to be created and realized, there needs to be collaboration from many separate individuals, it also shows the importance of knowing where your rights are. Like many successful creatives, Gaiman has been subject to his fair share of copyright issues. That is just going to be a reality of engaging in any sort of creative endeavor. Here, however, he demonstrates the strength of knowing what capacities you have, and what rights you possess.
This finally demonstrates that he continues to learn and develop his skills as he goes. Neverwhere was Gaiman's first television show that he wrote extensively for. By working with others who were more competent than he was, he was able to learn quickly and effectively how to adapt his own skills, writing for comics, novels, and short stories to writing for television.
Where his previous novels and other works started to develop Gaiman's trademark style, Stardust is a sharp left turn into, by Gaiman's account, pre-Tolkein English fantasy. The story fits this description entirely - instead of relying upon the tropes established by Tolkein about Elves and Dwarves, he builds upon the older traditions of the fae and myths of the isles serving for a backdrop for the world.
The biggest lesson that a writer can learn from Stardust is the importance of continuing to develop your voice and your craft, ensuring that you remain bold enough to go out into untold directions instead of re-hashing the same old stories that you've already written. The story was originally conceived as a book with pictures, and there had been talks for it to be published as a sort of comic book with DC, and originally it was released in this way. It did not reach success, however, until it was grown and developed into something larger in the novelized, and occasionally novel with plates, form.
This is considered by some, myself included, to be the piece that took Gaiman from a well known and successful writer to his current status as the rockstar of the literary world. There have been a number of instances where Gaiman has talked about the process of coming up with American Gods, often saying that it started more as a feeling than an actual idea - he knew that gods, and America, and coin tricks were important. Then he sort of sat on it for a while, then started to chip away at it. Gaiman describes writing the process, including interludes, as a process of discovery, implying that for at least this book he is a pantser rather than a plotter. He would write the interludes, the short stories about other people in the universe, with each sitting. The short stories about other characters were things he did while his brain figured out what happened to Shadow next.
This shows one of the biggest lessons that is weird to learn - don't write anything until you are ready to do so. Many authors, especially early ones (and I've seen this happen to myself) start running full-tilt into an idea before it is ready. And, at first, it works. But as you continue to delve in deeper and deeper to it, you see the problems that start to arise. The story moves slower and slower as you have to figure things out. If its bad enough, you just get stuck and can't move forward.
One of the most interesting things is the re-use of imagery and themes between Sandman and American Gods. Both have "shabby old gods", in Gaiman's terms, and focus on themes of belief, power, cons, and other similar things. Both reuse many characters in slightly different interpretations. This isn't a bad thing - a lot of authors do this and its just a matter of how much/how well you can hide it. I've heard a few authors point out that Orson Scott Card's work essentially loops on itself and is telling almost the exact same story at the end and the beginning.
Finally, American Gods shows the importance of marketing and social media. The awards and critical acclaim show it to be one of Gaiman's best works, but the social media and marketing are what helped ensure that people were able to find and fall in love with the world Gaiman has created here.