If you're looking for academic sources I can help a little. Wiedemann, The Julio-Claudians, 1989 is a good general overview. Eck, The Age of Augustus, 2007 is a good overview of the establishment of the Principate. For the army in this period, Le Bohec, The Imperial Roman Army, 1989 is a fairly standard text. Well-regarded books focusing on individual emperors include Levick, Claudius, 2015? and Seager, Tiberius, 2005. In fact, I'd just check out the entire Blackwell Ancient Lives series. For more general histories of the period I'd also look at other works by major publishers such as Potter, A Companion to the Roman Empire, 2006, Goodman, The Roman World 44 BC--AD 180, 2012, etc.

Particularly relevant primary sources for the Julio-Claudians are the Annals by Tacitus, the Lives of Suetonius, Roman History of Velleius Paterculus, and Cassius Dio, books 52-63. All of these texts are available for free on LacusCurtius.org.

Unfortunately, I cannot be of much help for non-academic texts. Of the texts above you'll notice some are pretty expensive, so there's nothing wrong with looking more closely at the cheaper ones, they're all good quality texts. If you want sources specifically to do with the Roman army then I can provide several more.