Frederik Pohl is credited with getting it all going, sort of. He was the young editor of Astonishing Stories and Super-Science Stories, nineteen when he started in 1940. His magazines were published by the discount line, "Fictioneers," of the Popular Publications pulp house...so he was getting less per week than the lowest-paid secretaries at Popular, and was expected to write for his own magazines. One way he did so was in writing book reviews...and, unusually for the pulps, he took his best shot at applying technical literary criticism to the books under discussion. Slightly younger member of the NYC-based sf-fannish group the Futurians (which included Pohl) Damon Knight took that as his model, for his reviews in fanzines and then for reviews in professional sf magazines, and so did James Blish, who chose to write his criticism under the pseudonym William Atheling, Jr. (after Ezra Pound's use of "William Atheling" for his own critical work...in its turn a reference to an historical figure among English royalty).
Donald Wollheim, one of the few members of the Futurians (slightly) older than Pohl (and the primary rival of Pohl's in the factionalism that developed in the Futurian Society) had published before Pohl, but became an editor afterward, and for an even less-well-bankrolled publisher...and his critical writing was somewhat less prominently published, as he focused on his editorial and publishing career. For that matter, Pohl never published a volume of critical writing, while Wollheim restricted himself to the survey The Universe Makers, a chatty review of broad themes published in 1971. Pohl has contributed to various anthologies of critical writing over the years, such as those edited by R. Bretnor, and had a column, "Pohlemic," devoted to criticism of literature among many other things as they occurred to him in the magazine Algol, later Starship, in the 1970s.
But Knight and Blish published books of their critical writing early on, Knight winning the first Hugo Award for nonfiction with the 1956 first edition of his In Search of Wonder, and Blish as Atheling, Jr. following up in 1964 with the first of what would eventually be three volumes of his collected critical writings, The Issue at Hand, both these volumes published by the small house devoted to sf criticism and historical writing, Advent: Publishers.
Advent is for the most part sustained these days by the NESFA Press, the New England SF Society imprint that grew out of the MIT-based fan group responsible for a wide range of Boston-area fannish activity, including the Boskone conventions, and the books in tribute to Boskone guests. Thus a relatively early NESFA Press publication, collecting the historical and critical essays of the third magazine editor to come out of the Futurians, Robert A. W. Lowndes, who published the contents of his The Three Faces of Science Fiction as editorials in his 1960s magazine Famous Science Fiction; despite some impressive reprints and new fiction in that Very low-budget magazine, Lowndes's essays were often the highlights of a given issue.
So...amidst a slow trickle, at first, of critical and historical works about sf beginning to appear in boards, beginning with Lloyd Arthur Eshbach's anthology of essays Of Worlds Beyond and J. O. Bailey's augmented PhD thesis Pilgrims in Time and Space in 1947, about half up through the early '70s had come from the ex-Futurians noted here...and these were among the most important books of my early reading...along with autobiographical and biographical works that these folks, and such fellow ex-Futurians as Judith Merril and Dave Kyle, would write and have published (most in the 1970s), along with those who were influenced heavily by the work of these folks (including Joanna Russ, to some extent Ursula K. Le Guin, Barry Malzberg, Robert Silverberg, Brian Aldiss and particularly Algis Budrys, all of whom produced their own collections of critical writings that have been covered by others and/or myself in this series of FFB posts). As Budrys would note in the 1970s, Blish brought a better literary (and, as a student of music and Shaw's criticism, critical) education to the task, but Knight was even better at stating clearly and forcefully his technical and other sorts of assessment of a given work. All, however, are valuable, particularly when compared to such lesser work in the same vein as that offered by Sam Lundwall and Alexei Panshin, among many others who followed the pioneers...or Sam Moskowitz, who attempted to do important work contemporaneously with these folks, and sometimes did so well, but usually didn't.
For more, and probably more amusing and fully-realized, examples of today's choices of books, please see Patti Abbott's blog.