The follow-up to Silverberg's earlier Arbor House anthology focuses on short novels, or novellas, and discusses the difficulty of reprinting notable works of these lengths in anthologies with limited space. (Silverberg did an earlier, paperback anthology, Great Short Novels of Science Fiction, that just preceded in 1970 the first volume of his Alpha series. That book included 3 of the 15 stories here.)
Though a companion to the earlier Arbor House anthology, Silverberg slightly relaxes that one's scope; one story here was published in 1945, and Silverberg's introduction mentions that one story was written as early as 1941.