Thank you everyone for your comments and your emails on the issue of sleep. I really appreciated them. It's nice to have stuff like blogs and facebook where you can ask other parents (en masse) for advice, etc.
We decided to implement the "cry it out" method. And it worked. After progressively less crying each time I put him down, Spencer learned to put himself to sleep. Now that we've been doing it for over a week, he does pretty good. Sometimes he still cries when I lay him down for sleep... and occasionally he revolts and refuses to sleep. The first and second naps usually go pretty well... it's the third nap, around 2:30 or 3:00 that gives us the most trouble. He still only sleeps for about 45 minutes at a time, but I think that's just how he is. He's what one book calls a "short napper". As long as he gets some sleep, everyone is happier. I don't dread naptimes as much, and I love the time that I spend rocking him. He cuddles right into me and I think (over and over) just how much I love him.
So again, thanks everyone. It's nice to know I'm not the only parent out there who's trying to figure it out as I go along! I remember hearing people joke about how they wish their kids came with an instruction manual... and now I know the feeling :) He's 5 months old today and I can't imagine life without him. Happy 5 months, Spencer!