While this book has taught us the essential skills of GameMaker, there is still so much more to learn. There are many websites available online where we can ask questions and learn more about GameMaker, and also how to achieve specific tasks inside of GameMaker.
The GameMaker community is one of the best places to ask questions and learn about GameMaker. Here, people ask questions on how to create deferent things, what things do, and how they can be used. More experienced user can then answer these questions.
YouTube is one of the best mediums through which to learn anything. There are hundreds of tutorials, showing how to create many different things in GameMaker such as turrets, role playing games, and so on. Here are some YouTube channels that have great tutorials for GameMaker: