Orphanage, by Robert Buettner

The first book of five military sci-fi, space opera series. I picked up and read Orphanage and got hooked pretty quickly. Comparable to some of the Elizabeth Moon books.
Jason Wander, the main character, starts out as a lowly corporal and ends up a general by the end of the book. Basically, mankind's first meeting of aliens, and it's not a good one. The Slugs park on Ganymede and start dropping large chunks of rock onto Earth's larger cities, totally destroying some of them. Earth's beleagured military sends an attack force to try to destroy the Slugs; only problem is it's been decades since any spacecraft have been needed, so it ends up being a patched up, roughly put together force with little chance of success. The main characters mostly end up hanging around through most of the rest of the series, so there is a nice continuity to the series. Easy, enjoyable, basic sci-fi.