The curve allows you to "hook" around as you are sewing. The photo at right should get the idea across.
Curved needles are pricier in the U.S. than are straight needles, at least as far as we've found. However, for those in the U.K., Shepherds Falkiners sells straight and curved for the same price. Not sure what the sizing numbers mean over at Shepherds, but in the U.S. we use the darning #1, which we use for sewing 3-ply (3 cord) linen thread and as thick as 6-ply. In the U.S., both straight and curved needles are available at Hollanders and Talas.
You can also make your own curved needles out of straight ones. How to do this is explained after the jump.
To make your own curved needles, you just need a candle, and two needle nose pliers.
Holding the needle with pliers on both ends, bend it ever so slightly as the needle starts to heat up. It only takes a few seconds.
An angle that is slightly larger than 90 degrees works well. Remember that the needle will be hot, so place it on a heat-resistant surface to cool down once you've bent it to the angle you'd like.
The needle will have soot on it so remember to clean it off with soap and water before using it to sew your nice clean pages.
A little trick to help in pulling the needle and thread through a tight hole is to grab the needle with a balloon. We keep a bag of water balloons on hand for just that purpose. That's unfilled water balloons, in case you were wondering (you never know). You may also find it useful to use a pair of small needle nose to pull the needle through.
Anything to add to the discussion? Please leave a comment.