The Force and Mathematics!
We know that between A New Hope and The Empire Strikes Back that Luke Skywalker's skill with the force increases. Heir to the Jedi tells how Luke begins to grow as a self-trained Jedi. Heir gives us Luke's journey shortly after the battle of Yavin through a first-person narrative. We get his thoughts, his confidence, his worries, and his growing connection with the Force. The story is self-contained, giving us a little bit of romance, some math and science, and exploration of the force as a construct.
This book was also a very entertaining read. The existence of a song known as "Vader's Many Prosthetic Parts" is a wonderful bit of flavor, and Luke's thoughts often made me smile. "I hasn't read the histories of those 'seduced' by the dark side, but I doubted that any of them had been corrupted by a vegetable of questionable nutritional value."
Luke is a young adult during A New Hope, he's a bit of a cocky kid who found life on the home farm boring and wanted more than anything to escape that life. Heir to the Jedi fills in some of his growing up, his transition from reckless farm-boy to focused Jedi.
Advanced Reader Copy copy courtesy of NetGalley; differences may exist between uncorrected galley text and the final edition.