I have been working with SQL Server for more than 8 years now continuously and I like to read a lot. Some time I read easy things and sometime I read stuff which are not so easy. Here are few recently released article which I referred and read. They are not easy read but indeed very important read if you are the one who like to read things which are more advanced.
SQL Server Standards Support Documentation
The SQL Server standards support documentation provides detailed support information for certain standards that are implemented in Microsoft SQL Server.
Microsoft SQL Server Protocol Documentation
The Microsoft SQL Server protocol documentation provides technical specifications for Microsoft proprietary protocols that are implemented and used in Microsoft SQL Server 2008.
Microsoft SQL Server Data Portability Documentation
The SQL Server data portability documentation explains various mechanisms by which user-created data in SQL Server can be extracted for use in other software products. These mechanisms include import/export functionality, documented APIs, industry standard formats, or documented data structures/file formats.
Reference: Pinal Dave (https://blog.sqlauthority.com)