Fair use designates only a restricted portion of the copyrighted material to be implemented within some educational-related means, such as 10% of a song in a multimedia presentation assessment, that will not be used for anything other than said educational purpose. I doubt that includes textbooks, but even so if you abided by the same conditions, you would legally only be allowed to read the first few pages or so.
The only other way you could get away with it is if the book came with an electronic copy that had been purchased by your school or institute with the declared intention of redistribution within the organisation only, which then could be shared with you as a student or staff. If the book has no electronic copy, then any copy you obtain is already infringing copyright, I'm afraid.