Among those in my acquaintance who are fond of Celtic art, George Bain's Celtic Art is the one which most buy first. I did. Then I read it and found that some of his instructions (particularly for creating knotwork) seem totally off-the-wall and totally indecipherable. However, for some reason, he seemed to be the only one who has ever come up with a way to recreate impossibly complex knotwork panels, and do more than just plain fretted knotwork. Fear not, Dear Reader! If you follow his instructions, practice the designs as he shows them, you will ultimately discover what he is trying to say. And when you do, the entire world of complex, bewildering and magical knotwork will open up to you.
However, knotwork is not the only type of art that Bain covers in this book. He also analyzes fretwork (mazes), spirals and zoomorphic forms (animal and human forms which can sometimes even be interlaced), as well. These, comprise the entire corpus of form used in the great books of Celtic and Anglo-Saxon art which were created in the 6th through 8th centuries.
Let me also put a good word in for Bain's survey of Hiberno/Anglo-Saxon calligraphy as well, in which he assembles many of the fanciful letter forms for which this form of writing is known.
I do own all of Aidan Meehan's books (and also recommend them), but if you can only afford to buy one book, buy this one. Then, as you can afford others, branch out into Meehan (and even Bain's son, Iain has written a book on knotwork!), and begin purchasing surveys of the Books of Kells and Lindisfarne so that you can study from the true masters.