Boswell is himself the subject of mountains of scholarship, not least on just how reliable he is as a biographer. Donald J. Greene is the leading Boswell-basher, in articles like "The Logia of Samuel Johnson," The Age of Johnson, 3 (1990), 1-33 (scholarly), and "The World's Worst Biography," The American Scholar, 62:3 (Summer 1993), 365-82 (less scholarly).
Boswell is hardly the last word on Johnsonian biography and in fact, he's not the first, either. Two of his contemporaries produced important rival biographies before Boswell: Sir John Hawkins's Life appeared as the first volume of Hawkins's 1787 edition of Johnson's Works. There is no modern edition of the complete text, but Bertram Davis did both a handy abridgment (New York: McMillan, 1961) and a critical study (Johnson Before Boswell: A Study of Sir John Hawkins' Life of Samuel Johnson [New Haven: Yale Univ. Press, 1960]). There's also Hester Thrale Piozzi's Anecdotes of the Late Samuel Johnson, which appeared in 1786, and can be read in Hill's Johnsonian Miscellanies among other places. And a collection of minor early biographies was published by O M Brack, Jr., and Robert E. Kelley as The Early Biographies of Samuel Johnson (Iowa City: Univ. of Iowa Press, 1974).
Modern biographies abound. Walter Jackson Bate's Samuel Johnson (New York: Harcourt, Brace, Jovanovich, 1977) is one of the best. James Clifford covers the early and middle years in two volumes, Young Sam Johnson (New York: McGraw-Hill, 1955) and Dictionary Johnson (New York: McGraw-Hill, 1975). The others are a mixed bag. I'm fond of Robert DeMaria's Samuel Johnson: A Critical Biography, but wouldn't recommend it to newcomers; it's more critical than biographical, and assumes you already know much of the life.
A great mountain of disorganized and undigested biographical information can be had in Johnsonian Gleanings, ed. Aleyn Lyell Reade, 11 vols. (London: Francis, 1909-52).
This is part of a Guide to Samuel Johnson by Jack Lynch. Comments are welcome.