The Moth and the Candle is probably one of the best biographies about James Boswell. It is well-researched and well-written, but with its 277 pages, it is far shorter than Peter Martin's A Life of James Boswell and the authoritative two-volume biography by Frederick Pottle and Frank Brady.
At the same time, it is still so relatively so as to include most of the modern research which has been done into the life of James Boswell in the 20th century, which makes it somewhat better than other well-written biographies of equal size from the 1940s and earlier.
The book also contains a number of illustrations (paintings and drawings) of Boswell and his friends and relatives.
The title of the book is a reference to a conversation between Boswell and Johnson, related by Boswell in the Life of Johnson as follows:
I teized him with fanciful apprehensions of unhappiness. A moth having fluttered round the candle, and burnt itself, he laid hold of this little incident to admonish me; saying, with a sly look, and in a solemn but quiet tone, 'That creature was its own tormentor, and I believe its name was BOSWELL.'
Although out-of-print The Moth and the Candle is usually widely (and cheaply) available via AbeBooks.