This is not, as some might initially expect, a guide to the persons in Boswell's journals - you have to stay on this site for that kind of thing. It does overlap, however, as it is a guide to about 350 persons mentioned in Boswell's Life of Johnson (1791).
The first "chapter" is called "January 1, A Day with Dr. Johnson" and it sets the stage for the rest of the book. Each page is a new date and a new person, and the author states his intention as follows: "Days with Johnson through a period of nearly forty years have shown the present writer the impossibility of becoming bored in the doctor's companionship. I hope a daily visit to Johnson throughout one year will show my readers the truth of what I now say, and help to answer the questions Who's Who in Boswell?"
All dates and persons are described on exactly one page, and all in all the book is very matter of factly and not particularly amusing or exciting, despite the author's intention to demonstrate “the impossibility of becoming bored in the doctor's companionship”. But then again, the book isn't about Johnson but about his many acquaintances, and as such it is a good and valuable guide to readers of both Boswell and Johnson.
Who's Who in Boswell by J.L. Smith-Dampier was first published in 1935, printed at the Shakespeare Head Press, St. Aldates and sold by Basil Blackwell, Broad Street. It was reprinted several times but has been out of print since the 1970s.
It is usually possible to find the 1935 edition for up to £80 at AbeBooks.