Before diving into the PDFs, it is important to understand why you are learning this. Azure Synapse Data Explorer is a fast, fully managed data analytics service for real-time analysis on large volumes of data streaming from applications, websites, IoT devices, and more.
Microsoft has shifted its documentation to be incredibly user-friendly. While they prefer web viewing, many official documentation pages offer a option in the top-right corner.
No official “Azure Synapse Data Explorer” eBook exists as a direct free PDF. However, these related free eBooks cover KQL and Azure Data Explorer (identical engine):