Complex sentence: Colfer had intended the series to be a trilogy, wrote more since the novels spawned more ideas, but decided that I could only write a book about Artemis if the story was strong enough, so I planned one at a time.
Split into simpler sentences: Colfer had intended the series to be a trilogy but wrote more novels since the series spawned more ideas. He then decided that I could only write a book about Artemis if the story was strong enough, so I planned one at a time.