"Why won't my sewn books open properly?"
"Why don't the pages turn nicely?"
"Why does the paper crack and warp when I fold it?"
"Why do my prints constantly jam the machine?"
The likely culprit is the grain of your paper running in the wrong direction.
I have some important points to make about commercially available paper, but before that, rather than reinvent the wheel, I'd like to direct you to an excellent explanation of testing and observing paper grain, and why it matters - Rhonda's post over at My Handmade Books, a Blogging buddy - has provided an excellent resource that you should read: My Handmade Books ~ Paper Grain
Now, to my Top Tip ...
When you buy paper in order to turn it into greeting cards, booklets, sewn books and the like, especially if it is packaged and you cant get to it to do the grain tests, there are some 'good-bet' general guides to use..
For art pads and sketchbooks - grain direction is almost always from top to bottom on the front cover (see picture).
That's because you are likely to use it in the orientation it comes in on the pad.
For reamed printer paper - grain direction is almost always running along the long side of the paper (see picture).
Known as 'long grain paper' - so it won't jam a printer machine as it goes through.
For folded signatures, booklets and cards, you'll be wanting SHORT GRAIN so it's running parallel to the spine. How do you get that?
FOR PADDED ART PAPER - choose Landscape orientation
FOR PRINTER PAPER - choose a size that is twice as big, and cut it down in half (eg A3 into A4)
..hope this helps unravel the joys and pitfalls of using readily available paper..