This is a button from 1986, promoting Pocket Books' Star Trek books. With this type of button, you simply fold down the tab and slide it onto your collar, shirt pocket, or anywhere else you'd like to wear it.
I added "bookmark?" to the title because I have seen it described as that. I don't think that was an intended purpose, and I didn't think it would work very well as one, but I boldy went and tried it out and found that it actually does work as a bookmark. You can just leave the very top sticking out of the book. I wouldn't recommend it, though, because it is metal, which could damage the pages of a book (or your fingers!), and because it is a little thick for a bookmark, and if you left it in too long it might cause your book to flip open to the place you'd "marked" even after you remove it.
|Well, the back is a little plain.|
- Pocket Books has indeed published a lot of Star Trek books. You'll find several of them on the Zach Trek Books page