I've done this about three times, expanded an idea into multiple books. However, a few people have told me that it shouldn't be a series unless it presents itself as a series. In other words, you shouldn't have to expand it. If it's series material than it doesn't have to be lengthened into a series(because it is already a series). Is that true? Am I doing this incorrectly? Another issue: If expanding an idea is perfectly fine, how do you personally spin a small premise into a series? Do you just keep brainstorming and adding on to what you have? I had a one-novel idea once and it ended up being three novels. It turned out perfectly fine. Nothing lagged or dragged on. Add: My mistake, guys! I meant "trilogy", not "series".