Osborne McGraw-Hill may be bringing the book in and out of print. When I got my copy in 1992, it was out of print but still available in bookstores, so if it just recently went out of print, they probably do infrequent printings and enough noise from potential readers may be sufficient to convince them to bring the book out of retirement.

Write to Osborne McGraw-Hill and let them know you want to buy a copy. Bookstores don't tell them when one person goes in and it's out of print, but publishers do sometimes listen when plenty of potential readers write to them. Their address is (or was when the book was published...)
Osborne McGraw-Hill
2600 Tenth Street
Berkeley, California 94710
U.S.A.

Good luck!