They're A5 size, printed on quality paper and delivered to your door, cheap, from the good folk at The Book Depository.
What I really like is that there's no fluff - just the pictures and captions, presented "photo album" style like this.
My growing collection sits on my bedside table. I find it very relaxing last thing at night to look through and ponder the images, undistracted by any text.
The old guy in this photo is Eugene Atget, a Frenchman who didn't take up photography until he was forty years old in 1897, but produced images with a strength and clarity that looks contemporary today. Way ahead of his time.