What kind of books are you looking for? For a general overview of early Islam and its conquests, Robert Hoyland's In God's Path: The Arab Conquests and the Creation of an Islamic Empire (2014) is your best bet. Another good introduction is the chapter from the New Cambridge History of Islam (2010), especially if you can access it from a university library. For a detailed analysis of military issues, check out Hugh Kennedy's The Armies of the Caliphs: Military and Society in the Early Islamic State (2001). If you are more interested in the social/political side of things, his The Prophet and the Age of the Caliphates (2004) is excellent as well. None of these books deal specifically with the Rashidun Caliphate, but for this period we have very few sources, so it's expected that there aren't any recent attempts at histories focusing on them in particular. The books I suggested though are all very recent and readable (Hoyland in particular), so you can use their bibliographies to find books focusing on your more specific interests.