Like I said, I am a prime member of Amazon and I am very happy that I became a prime member. If I need to buy something (not just books, but almost everything else), I go there before any place else. If it's a prime item, I know I can get it in two days with no shipping charge so that is a very good reason for me to order from them. I do go check out other sites for price comparison, but often times after the shipping/handling charge, it comes out about the same or Amazon being better. Even it the price is slightly better with non-Amazon, there is no guarantee of delivery in 2 days (2 days is often the case if the shipper is in the same state as you or in the adjacent state and they ship their stuff immediately, but that's just your hope... some places take time to just ship their stuff out.) unless you pay some hefty charge for it, so it's a no brainer to go with Amazon for me.
If it's not a prime item, the 2nd day shipping or free shipping is out the door, so then I do the normal price comparison with these items. Only upside to order via Amazon is for the reward points and that doesn't weigh heavy on my decision scale (If it is shipped from the Amazon facility, it's 3 points for a dollar worth of purchase; if it is shipped from a non-Amazon facility or go to Safeway and use your card then it's only 1 point for $1... or something like that. I think you get 2 points for gas purchases and some other things, but I am not sure on the 2 point thingy...) You get $25 Amazon gift certificate when the total reward amount reaches 2500 points.
I love it. I even bought their stocks a couple of years ago...