The World's Wild Places is basically an extension of The American Wilderness. The World's Wild Places was published by Time-Life International in Amsterdam in the early 70's while The American Wilderness was being published in the US, and the volumes look exactly alike; in fact I often confuse one series with the other. Be careful if you're buying these on e-Bay; people will often mix the two sets together (which isn't an inherently bad thing; you can buy both at once).
Like The American Wilderness, these volumes are pretty inexpensive, but hard to find on Alibris or Barnes & Noble. Try e-Bay first; you can often get a whole set of these for under $20. Most of these aren't listed on Alibris or Barnes & Noble at all; but you might get lucky.