Jeff Schewe's book The Digital Negative: Raw Image Processing in Lightroom, Camera Raw and Photoshop is excellent; there is a companion The Digital Print: Preparing Images for Printing in Lightroom and Photoshop. They are more books about whys and wherefores than software manuals. Victoria Bampton's Lightroom FAQs is good for a more which-button-does-what account.
They are physical books, which for reference I find easier to use, although they come in Kindle versions, which I also have but read on an iPad.