A common question from beginning bookbinders is if silk embroidery thread or regular cotton sewing machine thread can be used to make books. I tell them "Yes, but don't expect the book to last very long." Linen thread is simply stronger, and will hold up better over time.
Traditionally, linen thread has been used in bookmaking because of it's strength and long life. It's also not as stretchy as other fibers which makes for a tighter binding. It does, however, stretch over time, so a tightly sewn coptic book, for example, will loosen up a bit with use.
We use Crawford waxed Irish linen thread for most of our books. We typically use 3 or 4 ply -- also referred to as 3 or 4 cord, rather than ply -- and would not recommend using anything thinner than that, except on miniature books. You can also use 6 ply, 7 ply or 12 ply. I like using the 6 ply or 7 ply for exposed sewing as it shows off the stitching much more than the 3 or 4 ply.
We use the 3 or 4 ply for book structures in which the sewing is hidden, as they allow the pages to be bound more closely together than if using, for example, 6 ply.
Our thread comes from Royalwoodltd.com which also ships internationally. If you want to purchase a smaller amount (say, 5-10 yards) of one color instead of an entire roll then you may oh so conveniently purchase smaller amounts from us through our shop at badgerandchirp.etsy.com. Alternately, Bookfindings also sells smaller amounts of linen thread.
Non-waxed vs Waxed Linen Thread
You may buy both waxed and unwaxed thread. If you buy unwaxed, you'll likely want to acquire a piece of beeswax with which to wax it with. The wax reduces friction to allow the thread to glide through much more easily as you sew and keeps the fibers from fraying. We prefer the convenience of waxed thread, but many others like the unwaxed, being able to control the amount of wax they apply as they run their thread over their piece of beeswax. And some binders simply like the process and tradition of waxing their own.
If you would like to wax your own thread, you can find a nice little tutorial to do so from SeaLemon.