In my opinion, the Antenna Theory two volume set edited by Robert E. Collin and Francis J. Zucker are the best books on the subject written so far. Only problem is that these two books are very hard to find even at college/university libraries. Part I of the volume is actually quite easy to find on eBay or on Amazon (from private sellers) and goes for around $200.00. Basic antenna theory is covered in Part I. Part II is the harder of the two to find and Part II covers in great detail exotic antenna topics such as surface wave antennas, leaky wave antennas, and microwave lenses. I have not seen Part II being offered online so far. I was lucky enough to find both books and now they are a part of my extensive microwave engineering book collection.