Kathy Sierra, along with Bert Bates, began developing the 'Head First' series of learning books. These books were characterized by their unique approach to teaching complex topics in a highly visual, engaging, and brain-friendly way. The development process involved extensive research into cognitive science and learning theory.

'Head First Java', co-authored by Kathy Sierra and Bert Bates, was published by O'Reilly Media. It became an instant success due to its innovative and effective learning style, setting the standard for the Head First series.

Following the success of Head First Java, Kathy Sierra and Bert Bates, along with other co-authors, expanded the Head First series to cover a wide range of technology topics, including design patterns, object-oriented analysis and design, servlets and JSP, and more. These books continued to be highly popular, cementing Sierra's reputation as a leading voice in learning and education within the tech industry.

During this period, Kathy Sierra became a sought-after speaker at tech conferences and events. She presented on topics related to learning, user experience, and how to make technology more accessible and engaging. Her presentations were known for their energy, insights, and practical advice.