ASP.NET Core is an open source and cross-platform web application framework built by Microsoft. It follows on from the hugely popular ASP.NET version 4, with significant architectural changes making it much leaner and much more modular. ASP.NET Core is a great choice for building highly performant backends that interact with databases such as SQL Server that are hosted in the cloud, such as in Microsoft's cloud platform, Azure.
React was built by Facebook in order to improve the scalability of their code base, and was eventually open sourced in 2013. React is now a massively popular library for building component-based frontends and works fantastically well with many backend technologies, including ASP.NET Core.
This book will teach you how you can use both these technologies to create secure and performant single-page applications (SPAs) on top of SQL Server databases hosted in Microsoft Azure.