The best place to start is George Willick's Spacelight. This site contains essential biographical data and bibliographies for a long list of SF writers who are now deceased, including many regular Astounding authors such as Isaac Asimov, Randall Garrett, L Ron Hubbard, Raymond F Jones, H Beam Piper, Eric Frank Russell and A E van Vogt, as well as John W Campbell himself.

There are also excellent single author sites for Isaac Asimov, Eric Frank Russell and A E Van Vogt - who, in their very different ways, are probably the three most characteristic of Astounding's regular writers. Also useful is the Icshi site, which contains comprehensive bibliographic data on Van Vogt (which helps to disentangle the many later revisions and reworkings of his early Astounding stories), and the Testerman site which includes a large selection of Asimovian quotes, organized under various headings.

An author who is not typical of Campbell's Astounding, although he had one of his most important early stories published there ("Impostor", in June 1953), is Philip K Dick. Like the best of the Astounding writers, Dick was a man of ideas, and I've compiled a list of metaphysical quotations from his novels similar to the list of Astounding quotations on this site.

There are excellent collections of SF magazine cover scans (including Astounding) at NooSFere and Visco. Comparisons of the British and US versions of the covers can be found at Jaunting the Aether.

By way of more general SF bibliographies, there is the Locus index to science fiction and the Archive of Science Fiction. The latter resource includes a directory of SF pseudonyms, which is invaluable for authors like Randall Garrett, who used numerous aliases in his magazine-published work.