This is one of the books on SQL 2012 that does a good job of explaining how things work without going too deep:
You should also pick up a book on T-SQL and query writing. Itzik’s book is very good, although the latest edition will cover a few bits not in SQL 2012:
The SQL Server community is very active on forums etc. and Microsoft also have free online training courses. You can also download SQL Server Developer Edition for free and learn using the example AdventureWorks databases. Management Studio is now a standalone project and id freely availabe. There is also a SQL Server Developer Tools (SSDT) which gives you a Visual Studio with just the SQL tools.
If you happen to be into Linux and/or Docker SQL Srever 2017 Developer Edirion is also available on those platforms as well.
Career wise you cannot go wrong learning SQL Server, it underpins many large business and corporate systems and jobs needing skills in it tend to pay well. I started working with it in 2004 and it’s been at the core of what I do ever since. Fortunately it’s got so many components and uses that you can’t get bored and no one is an expert in all aspects - people tend to specialise in the roles that interest them most e.g. Database Admin, Database Developer, BI Developer and Data Platform Architect are just four of the roles where I work that deal mostly with SQL Server and the newer Azure based SQL products.