Hi there,
while there is no specific "how to learn HANA programming" book out there that would cover every aspect of this meanwhile massive platform (covering classic SQL, information model development, XSJS, UI5, the PAL/AFL/BFL libraries, GEOSPATIAL data handling and text analysis), I'd say actually going through the developer guides at SAP HANA Platform – SAP Help Portal Page is a really good idea.
Understanding the SQL syntax is relatively easy and there are plenty of web based, free tutorials around. So, go wild on these ones. Most of them will work on SAP HANA.
The more challenging but also more important part would be to understand SQL database programming on a conceptual level.
For that, there are indeed really good books available that cover a lot of things, you probably won't find that easy or discover yourself.
Before listing them again here, have a look into this discussion instead Books/ practice problems/Practice website for SQL.
Cheers
Lars