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• Microsoft SQL Server 2012 T-SQL Fundamentals by Itzik Ben-Gan
• Microsoft SQL Server 2012 Internals by Kalen Delaney
• Pro SQL Server 2008 Fail-over Clustering by Hirt, Allan

I wish these books were updated for each SQL Server version especially SQL Server 2017 as all were great reference books that were referred to many times.

You need at least one good reference book for T-SQL language coding and techniques, SQL Server Internals and SQL Server High Availability / Disaster Recovery (HADR).