The book of Gonzalez, Woods is surely the most widely accepted. Another excellent book that contains more advanced material is the "Image Processing. The fundamentals" by Maria Petrou.
Digital Image Processing by Gonzalez is the best. You can get the electronic version from here: http://lit.fe.uni-lj.si/showpdf.php?lang=slo&type=doc&doc=dip&format=0
the theory is not too much deep but it gives enough introductory insight.
you should also check the complete list here
http://opencv.org/books.html
if you're looking for a more theoretical book you should head for the gonzales as said by others, but i would also recommend you this, really well done in my opinion:
Recently it has been published the new edition of the "Principles of Digital Image Processing" by Burger & Burge for undergraduates: http://imagingbook.com/books/englisch-edition-3-vol-softcover/