If you are a student in MatE 273 at Iowa State University, the best resources available to you are first and foremost the lectures, which you should attend, but in addition the office hours held by the TAs and instructor are incredibly helpful. The TAs are literally paid to answer your questions about concepts you don't quite understand (also they get lonely when nobody visits), and the instructor's office hours are times that she or he has intentionally set aside to help you.
More generally, the book Materials Science and Engineering, an Introduction by William D. Callister (ISBN: 9781118324578) is a great introductory textbook for Materials Engineering. It is used at Iowa State University as the textbook for all of the introductory materials engineering courses.
The ASM Phase Diagram Database is a great resource that contains just about every phase diagram you could ever want. If you are on Iowa State's campus, you have free access to this database here: http://mio.asminternational.org/apd/index.aspx.