UbuWeb is a really amazing resource for anyone interested in 20th-century art and culture. It’s also got some interesting material for those interested in magazines.
You can look at volumes 1-3 of the really important Dutch magazine De Stijl in pdf format. They’re surprisingly plain and text-heavy, but I was stuck by the presence of ads in some issues.
Then there’s the incomparable Aspen magazine–the experimental magazine that ran from 1965-71, not the one for vacationers. I don’t want to spoil the surprise too much if you’re not familiar with it, but it’s pretty fascinating in the ways it with format and design. And what a roster!: Warhol, Beckett, Cage, LeWitt, Sontag, Barthes, Reich, Ballard, Richter, McLuhan, and many, many more of the artists, writers, and thinkers who defined the 60s and 70s. I’ll just say it’s well worth a look.
Finally, UBUWEB aslo has the first 3 issues of Dada magazine, edited by Tristan Tzara.