I can't recommend any text books as most of the stuff I get is off the net, or second hand books.
In the realm of earth sciences, James Lovelock wrote a book called Gaia. It's written at a fairly accessible level, and whilst I'm not fully convinced he presents his theory well. As a controversial theory it's an interesting place to start experiencing viewing arguments critically, an essential skill in science.
On the area of oceanograpy i´d suggest: "Invitation to Oceanography" by Paul R. Pinet. It covers the basics of oceanography and it proved quite usefull to me during the first and second year for me.
I´m more into the management side of it all but it´s a fine place to start and easy accesible.
this is the website that is used for background info in the book: http://www.jbpub.com/oceanlink2e/ [Broken]
The site also has an glossary cover in all the basic terms in oceanography.