Johnson's Prefaces, Biographical and Critical, to the Works of the English Poets (familiarly known as the Lives of the Poets, but pay attention to the actual title), originally appeared between 1779 and 1781 in the format their title suggests: as prefatory material to a large collection of the works of around fifty poets. They were first collected together in 1781.
Most of the Lives can be divided into three sections: a biography (usually collected from other sources; Johnson did little original research); a brief "character"; and a critical section, in which Johnson considers all of the major works of the author in question. These critical sections provide some of Johnson's most extended literary criticism.
Although most of the Lives were written especially for the collection, Johnson's Life of Savage had originally been published in 1744. Johnson knew Savage well in the years after he arrived in London, and that intimacy contributes to the great difference in tone between Savage and the other lives (to say nothing, of course, of the decades that separate their writing).
With over fifty poets (all men, incidentally) drawn from the years between the Restoration and the 1770s (no living poets were included), some of the figures are pretty minor: Yalden and Pomfret, for instance. Note, though, that Johnson chose only a few of the poets to be included; most of the editorial decisions were made by the booksellers who organized the edition.
Apart from Savage, the Lives that have received the most attention tend to be those of the most important poets: Cowley (Johnson's Life of Cowley helped to popularize the term "metaphysical poetry"), Milton (Johnson attacked his politics as those of "a surly and acrimonious republican" and had scathing things to say about Lycidas "easy, vulgar, and therefore disgusting" but he recognized the greatness of Paradise Lost), Dryden, Addison, and Pope. The Life of Swift, one of the weaker Lives, gets comparatively little commentary in spite of its famous subject.
For a full century, the only complete scholarly edition was The Lives of the Poets, ed. G. B. Hill, 3 vols. (Oxford: Clarendon Press, 1905). It has finally been replaced by The Lives of the Most Eminent English Poets: With Critical Observations on Their Works, ed. Roger Lonsdale, 4 vols. (Oxford: Oxford Univ. Press, 2006). Another edition in the Yale Works is promised, and the Yale and the Oxford editions will likely compete, but for now the Oxford Edition is the one to trust. Selections also appear in many anthologies, which focus especially on Savage and the critical sections from a few major lives Cowley, Milton, Dryden, Pope, Addison, and sometimes Swift.
A separate edition of The Life of Savage was edited by Clarence Tracy (Oxford: Clarendon Press, 1971), which pays far more attention to textual questions than Hill's old edition did.
The criticism in the Lives is central in most considerations of Johnson as a critic, so works like Hagstrum's, Keast's, and Hinnant's are good places to start. For the biographical side of things, see Folkenflik.
This is part of a Guide to Samuel Johnson by Jack Lynch. Comments are welcome.