Behind the Walls of Terra

I first met The World of Tiers in short stories as a kid - possibly published in Analog, I'm not sure - and they grabbed me immediately. The concept of jumping between worlds, taking on different personae, was wonderful for a teenager.

Later, as a young adult, I discovered the books - and immediately and obsessively collected them all. Farmer develops the story really well through the series, and I must go back and read them again. Now!