ART: Affordable art is the best kind, so good thing there's a whole fair dedicated to it. Today 70+ artists worldwide will be selling their work at prices ranging from $100 and up. Get something to cover up those bare walls and support artists all at the same time. Noon to 5pm // The Altman Building [125 W 18th St] DISCUSSION: The Strand Bookstore just turned 80 years old, and it's not a surprise it's......