I just re-organized my books the other day and noticed that I now have 20 hard back 4E books. 20! That's a huge number of RPG books. I don't think 1E or 2E put 20 books out total each. And this is just in 1.5 years. What will it be like 5 years from now, even it it all goes totally online or electronic??? It will probably be a massive amount of material that even many of the hard core gamers here on the forums will be totally swamped by, let alone their players.
I can't speak for 1e, but I actually wouldn't be all that surprised if 2e was the largest edition of D&D ever, in terms of number of books (as long as we disqualify all the 3rd party material for 3.x, which would probably put 3.x far in the lead).
You had the basic 3 (PHB, DMG, MM)
You had numerous additional MMs (don't know how many, but I have a Monster Manual Annual Volume 3, so I can say with certainty that there were
at least three of them, certainly more)
You had all the Complete Class books (Complete Wizard, Complete Priest, Complete Psionics, Complete Druid, Complete Ninja, Complete Shaír's, etc.; we're easily looking at a
minimum of 11 of these)
Then there were the Complete Race books (Complete Elf, Complete Book of Humanoids, etc.; at least 4 of those)
There were the books oriented towards DMs (Creative Campaigning, Complete Villains, Arms and Equipment; which makes 3 that I know of though there were almost certainly more)
The Campaign Sourcebooks (A Mighty Fortress is the only one I own, though there were definitely more)
The Options books (High Level Campaigns, Combat & Tactics, Skills & Powers, Spells and Magic; so at least 4)
Other books that weren't part of a "series" per se (Tome of Magic, Book of Artifacts, Encyclopedia Magica, Wilderness Survival Guide, etc.; so a minimum of 4 more)
Campaign Settings (FR, Dragonlance, Council of Wyrms, Planescape, Domains of Dread aka Ravenloft, Mystara, Dark Sun, etc., each of which had more than 4 sourcebooks such as: Monstrous Compendiums, Volo's Guides, Glantri, etc.; so easily more than 35 books there alone)
That's a total of 68 books, and I know I haven't covered everything that was put out for 2nd (heck, there are several from my own collection that I didn't bother to include, like Warriors of Heaven). I'd guess that 2e had easily over 100 different sourcebooks.
I'm not saying that 20 books in 2 years isn't a sizable amount. However, I seriously doubt that 4e will ever rival 2e in terms of number of books.