For books i WANT to read: I don't. I just go to the bookstore and wander.
I come across stuff ALL the time that "Oh! I've been meaning to read that." but if I don't have the cash in hand (because I have another book I want to read more already in arms), I don't buy it and forget again for awhile until I have the money/time to wander the aisles again.
How I keep track? I own them. I ran into a BIT of a problem with a basement flood awhile back (lost 17 garment box sized book boxes - that hadn't been unpacked, because I'm DIY'ing my house, and a LOT of stuff stays packed in the basement). Thousands and thousands of dollars worth of books. HIGHLY vexing. Esp. the text books at a couple hundred a pop. Fortuantely MOST of my beloved books were spared (I reread books). A flaw in the insurance (because this is my first house, so I didn't know better) meant that everything we lost in the flood wasn't covered. ((ACTUAL floods... like feet of water overflowing from a river... we were insured for, water damage from pipes we were insured for. Not rain and a busted drainpipe. Aaaargh)). But aside from the drown and mouldering books (about 1000-1500 in total), I own the rest (but I give away what I didn't like at all). Libraries are the bane of my existance... since they never have the ENTIRE series, and there's always a wait / 'in transit', or they don't have the books at all yet (waitlist for months).... and then I finally GET the book, read it in an hour, and forget to return it racking up $20 in fines. Ugh. Libraries. HATE them. ((Love them for school, not for pleasure)).