I've been trying to get DS to say more words. I'll repeat a certain words for days, and he just looks at me like I'm crazy and continues to just point and say "guh."
Today, I was looking for a birthday card, and DS was parked in the cart by some cards with pictures of animals. I wasn't really paying attention, but I hear DH say to him, "That's a rat" and repeats "rat" a couple of times. (It was actually a hamster, but whatever.) So five seconds later, DS starts saying "rat." Seriously. DH says something twice, and DS starts repeating it right away.
I'm assuming it was easier for him to pick up "rat" because he already knows "cat" and "hat." Right?
And coincidentally enough, DS is now sitting in the middle of the floor playing with toys, randomly saying "rat." Of all the words for him to pick up.... Sigh. I just want him to say "book" instead of pointing and grunting. Is that too much to ask?
Re: New words - semi-vent
It could just be that you're asking him to say words that he's not ready to say. There's an evolution to human language. You can google search it. The sounds that are easier for kids to say will dictate which words they handle first.
Off the top of my head, the sounds that you make in the front of your mouth (ie- M, D, B, etc) will be the first words your kid masters followed by words that you make in the back of your mouth (ie- G, L, K/C).
So, consider that when you're frustrated. 
  
I've read a little about easier/harder sounds, so I don't push him on those. I just want him to say "book" - I would accept "boo" since he can't do c/k (cat = "at"). "Book" is the most aggravating one since I know he can make a B sound ("ba" for ball, "beebee"/"baby").
Heck - I got him to say something resembling "orange" (he either says "oh oh oh" or "oh-at") when he kept demanding more tangerines So "book" seems so simple compared to rat and orange.
 So "book" seems so simple compared to rat and orange.