This link contains many Ebooks published by Microsoft that can be downloaded for free: Introduction Data Science, working with Microsoft Power BI, advanced content in SQL Server 2014, 2016, Azure, and more.
At these links: Part 1 , Part 2, You will find another collection of Ebooks in many topics such as: SharePoint, Visual Studio, SQL Server 2012, Azure and more.
Another collection of free books is available here. These books were published by Redgate and deal with many topics such as: development in the SQL Server environment, ADMINISTRATION, .NET, and more.
A list of Ebooks that includes a wide range of topics: Data Science, Artificial Intelligence, Big Data Architecture, and many other categories.