This paper proposes DeepWriter, a customizable, multimodal long-form writing assistant tool to overcome the limitations of large-scale language models (LLMs), which struggle to be utilized in specialized fields such as finance, medicine, and law. DeepWriter leverages an offline, refined knowledge base to perform tasks, outline generation, multimodal information retrieval, paragraph-by-paragraph writing, and review. It leverages both textual and visual elements from a structured corpus to generate factual, consistent, and expert-level documents, while hierarchical knowledge representation enhances retrieval efficiency and accuracy. Experiments on financial report generation demonstrate that DeepWriter outperforms existing methods in both factual accuracy and generated content quality.