Yeah, recommended readings are not worth the purchanse-if you get stuck just seek them out from the library or even from your lecturer (a lot of lecturers are generous with lending out their books!), as you will probably only need it once or twice at most. But even a lot of required books turn out to be a waste of money...I have a couple of so-called "required" books from 1st year that to this day I have only flicked through.
Unless the book is something you will refer to over the whole course (such as a dictionary or a general book on physiology or something) or something that will still be a useful reference when you graduate, then you may be able to do without it.
Some books may be very helpful while you're doing that subject, but you may "grow out of it" at the end of studying that subject, and by the time you've graduated everything in the text book is more or less basic knowledge that you won't need tp look up. In this case, you can weigh up whether the cost is really worth it for only one a semesters use, or whether you can get away with just borrowing it from the library/off a friend. Other books turn out to be a waste of time altogether, so try to look through the book first or ask around the higher years to see how useful they found it before you buy. A good indicator of useless books is if a lot of people seem to be selling the same book...if lots of people are getting rid of it, it mustn't have be that useful.