The Cognitive Stack in Big Data and AI in the Agentic Era is a visionary guide to understanding the next frontier of artificial intelligence. Moving beyond generative models, this book introduces readers to the rise of agentic AI-systems that don't just react but act autonomously to reshape industries, research, and everyday life.At its core lies the Cognitive Stack, a layered framework that explains how data flows, transforms, and powers intelligent agents. From the foundations of big data pipelines to the analytics engines, generative intelligence cores, and action layers, the book provides a clear mental model of how autonomous systems "think" and execute decisions. Through real-world case studies in healthcare, finance, creative industries, and disaster response, readers will see how agentic AI is already revolutionizing practice. Ethical challenges, transparency, and human-AI collaboration are addressed with nuance, ensuring that innovation is balanced with responsibility. Whether you are a researcher, technologist, business leader, or curious learner, this book offers both conceptual clarity and practical blueprints. With diagrams, workflows, and hands-on guidance for building your first cognitive stack, it is not just a roadmap but a toolkit for thriving in the agentic era.