For Justinian, the most accessible book is probably still J. A. S. Evans' The Age of Justinian: The Circumstances of Imperial Power (1996). If you want something a bit more academic, you might also want to try out The Cambridge Companion to the Age of Justinian (2005), if you can access it from a library. I would also like to recommend the very new biography of Empress Theodora, which naturally talked a lot of about Justinian: David Potter's Theodora: Actress, Empress, Saint (2016). It is a wonderful retelling of the story that fully integrates many of the advances made by scholars in the past few decades, providing an up-to-date view of Theodora that is much more nuanced and interesting than others (which still rely on the polemical words of Procopius, a problematic source that has to be used very carefully). Moreover, Potter paints a wonderful picture of what Constantinople and the empire were like in the sixth century, of how the average person might have lived and the sights they would have seen, aspects that are missed out it from more traditional approaches to this period. There are a few errors, but they are dwarfed by how readable and interesting Potter's prose is, so I highly recommend this book!