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This is so off-topic, but dang if that picture does not look suggestive...

That was going to be my comment as well.

Probably why the actual cover is an offset crop.

Would be interesting to know what got changed. I have the paperback first edition and if this are just minor revisions it would be nice to know

The Amazon links at the bottom of the post go to the first edition of the book; the second edition seems to not be listed on Amazon yet.

Looks like the hardcover and the ebook updated but not the paperback edition.

Mastering Nim, 2nd Edition