Red Gate has dozens of short, focused very technical books available, and most offer free downloads of the PDF version.
Dozens of great bloggers including lots of SQL Server MVPs at SQLBlog!
Check out the blog posts by members of the SQL Team at Microsoft. Lots of great stuff, include lots of in-memory OLTP information!
Awesome SQL Server Tutorials on a wide variety of topics. They all start from the basics and are all Hands On!