Do you know if it's a first printing? Apparently the first printing sold out (& is probably pretty rare). Then there was a second printing in 1977 or 1978 that they must've printed a TON of. I say this b/c a few years ago I went to the "Star Wars: Where Science Meets Imagination" Exhibit at the Saint Paul Science Museum. They had a decent stack of these second printings for sale in the gift store. They were uncirculated, but in varying states of condition (though most were pretty nice). I actually bought one b/c it was so cool (& such a surprise to see real vintage merch for sale at an official LUcasfilm exhibit like that!). But they were only about $20 or $25 if I remember, so probably not worth too much (especially considering they were probably a little overpriced like everything else in the gift shop...).

So yeah, if it's a first printing, it might actually be worth something, though I couldn't really tell you what.

If it's a second printing, it's not worth much but still a really cool collectable.