Short answers, pulled from the story.
The J. R. R. Tolkien Companion and Guide is a 2006 reference book by Christina Scull and Wayne G. Hammond. It provides a detailed chronology of Tolkien's life in volume 1 and a reader's guide in volume 2.
The J. R. R. Tolkien Companion and Guide was written by the husband and wife team of Christina Scull and Wayne G. Hammond. The pair had earlier published The Lord of the Rings: A Reader's Companion in 2005.
The first edition of The J. R. R. Tolkien Companion and Guide was a two-volume work published in 2006. The 2017 revised edition expanded to three volumes occupying 2720 pages, with the Chronology volume alone reaching 936 pages.
The J. R. R. Tolkien Companion and Guide covers people, places, organisations, biographical topics, literary topics, and writings by Tolkien. Its preface states that it is not a handbook of his invented lands and characters, so it avoids entries on fictional figures of Middle-earth.
David Oberhelman in Mythlore called The J. R. R. Tolkien Companion and Guide undoubtedly a seminal if not the definitive reference work on the Professor. John Garth in Tolkien Studies described it as a super-heavyweight contribution and best on biographical matters.
The 2017 edition of The J. R. R. Tolkien Companion and Guide expanded to three volumes, extending the Chronology to 936 pages and splitting the Reader's Guide into Part I for A to M and Part II for N to Z. Jason Fisher noted that defects and oversights were fixed, including running headwords and a list of topics covered.