SQL Server has tons of material available and in itself is a daunting task to read through and with so much happening and the changes that are being pushed on constant basis makes it near impossible to keep yourself update.
Some of the reference material that I have used over past few years are as follows.
Books:
Benjamin Nevarez’s books on query tuning are more than worth their price. I have started reading those to help me understand this aspect of SQL server and they are absolutely awesome.
Itzik Ben-gan if you are a just starting with SQL server and want to know about internals of SQL server development, Books by Itzik are a must
Kalen Delaney’s any SQL Server Internals Book , Latest is about SQL server 2016. Kalen’s books have helped me understand architecture of SQL Server and provided me with basic understanding of the Internals.
Blogs: There are tons of blogs out there but few I look in case I am in a soup are
Forums:
Forums are a great place to look for troubles or issues people are facing and sometimes are a great help in case you face similar issue.
Learning Websites:
- Pluralsight
- Microsoft Virtual Academy
- edx
- MCM Videos: Though they are for SQL Server 2008 but still the are awesome and are a great help to anyone willing to deep dive into SQL Server basics and Internals.
- BOL : Microsoft has all the documentation that is required for SQL Server and it is best place to start but current web search will land you on pages referencing to latest version of SQL Server i.e. SQL 2016 and SQL 2017 for older version use the links referenced with the SQL server version
Saurabh Srivastava's SQL Server Blog 