Tom Shippey is one of the leading scholars writing about Tolkien. He's a Medievalist, and in fact has held the same position as Tolkien.

He loves LOTR and The Hobbit, and is deeply versed in both Tolkien's inspiration in Medieval literature, and Tolkien's own writing.

And Shippey's knowledge and love shine.

Tom Shippey's Tolkien Author of the Century is one of my favorite books about Tolkien by one of my favorite writers, and it's currently on sale as an ebook at pretty much all the major ebook retailers for 1.99.

Here's an AW affiliate link to the Amazon page for Tom Shippey's Tolkien Author of the Century.