Unfortunately, there are not many materials in the form of textbooks available that are explicitly dedicated to explaining in-memory computing and in-memory databases in detail. That's why for a moment most valuable and usable sources of information in this domain are mainly blog articles, online courses, source code documentation, and scientific publications. Please, find below links to some online courses (free once) and other useful materials. A brief overview of technologies can be found in the following post Innovations of In-Memory Data Management.

SAP HANA (online courses)

• In-Memory Data Management 2015
• In-Memory Data Management In a Nutshell
• Introduction to Software Development on SAP HANA
• Next Steps in Software Development on SAP HANA
• Software Development on SAP HANA (Delta SPS 09)
• Software Development on SAP HANA (Delta SPS 11)
• Software Development on SAP HANA (Update Q4/2016)
• Text Analytics with SAP HANA Platform

Try SAP HANA

• SAP HANA Cloud Platform
• SAP HANA Express

Apache Spark (online courses)

• Big Data Analysis with Apache Spark
• Introduction to Apache Spark

Try Apache Spark

• You can use your own installation from an official repository (as long as it's open source)
• Sandbox from here Hortonworks

Other "In-Memory" field players (there are other ones as well)

• https://www.voltdb.com/
• http://www.memsql.com/
• https://www.gridgain.com/

Collection of various materials on in-memory computing

• In-Memory Databases on ODBMS.org
• In-memory DBMS by Curt Monash

I hope that the above list of resources could help you to improve your awareness about in-memory computing and databases.