I think there is a mistake (circled in red) in the following figure in my book:
http://img230.imageshack.us/img230/5223/figure.jpg

Isn't the positive sign supposed to be written over C as it is an electrophile and not O? If I am wrong, then please explain.

Thanks!

I think you are right. The oxygen cannot be positive here, and the carbon has to be positive to attract the nucleophile.