Alert - I am a very senior Oracle "systems" DBA. I am not a developer or an App DBA. So I read and reviewed this book as a systems DBA (install, upgrade, migrate, OS tune, DB tune, I/O tune, B/R, DR) point of view.
It covers the features Oracle wanted you to hear about and some you did not. It is well-written, but its coverage of Pluggable Databases (PDBs) and Container Database (CDB) is light for my taste, along with the coverage of the performance-related changes. PDBs and CDBs AKA "multi-tennant database architecture" is a big topic and could fill a large book on its own. Most will consider PDB/CDB "the reason to go to 12c", so you should get more info from Oracle and non-Oracle sources before you plan your real PROD upgrade.
Other topics include Flex Clusters, Flex ASM, RMAN's ability to recover a table, Data Guard's ability to use a stripped down instance as a "repeater", and SQL's new Identify Column and Invisible Column. Also new features in PL/SQL, BI, DW, and Security. The big news in Manageability is the replacement of "DB Control" with the new "DB Express" which is less powerful, especially re: RAC, but has a smaller resource footprint. (*NOTE* if you do not now use Oracle Enterprise Manager 12c Cloud Control - you need to do that ASAP)
I liked the book and recommend it be in your library of 12c books. I especially liked page 55 that had a nice lifetime support date diagram, giving you a big-picture glance at the road on Oracle releases, versions, and overlap . For those DBAs who do upgrades a lot, it reminds us all how many times we have done this complicated task in the past - and when.