These are the books which are not “by Tolkien” but which I imagine virtually every Tolkien fan owns:

1977. J. R. R. Tolkien: A Biography, by Humphrey Carpenter
1978. The Complete Guide to Middle-earth, by Robert Foster
1981. The Atlas of Middle-earth, by Karen Wynn Fonstad

Do you agree, and/or are there any other books which aspire to this level of ubiquity, utility, and high regard?