The books are not presented in order or importance, just five books.
1. Aikido and the Dynamic Sphere by Adele Westbrook and Oscar Ratti
If aikido was a college class, this would be the textbook for the class. It gives you a deep enough piece of everything that makes up aikido to start to put them all together. There is still enough left unsaid to make you curious and want to delve deeper into the art and philosophy. However, if you are a beginner there are few books that can be more useful than this one.
Also, the illustrations! I cannot stress how much I love the illustrations in this book, and its companion Secrets of the Samurai. The simple, fluid, black and white drawing given a sense of movement that's required when trying to illustrate for aikido. The book would be worth it if it just had the illustrations. The fact that there is really good information, and a lot of it, makes it even better.
4. The Way of Peace-Ueshiba MoriheiIf you are looking for more on the philosophy/spiritual side of aikido, then this book is one of the most important for you. These are (supposedly) the words of the founder when discussing what aikido is. I only say supposedly because there is some ideas that it was the words of O-Sensei, but because he wrote and spoke in a combination of mystical speach, and in an old fashioned manner, that his son, Ueshiba Kisshomaru, re-wrote the pieces to be more readily digestible. If this is the "easy" version, I really would hate to see the first draft. The book is a series of one sentence to one paragraph addresses of what aikido is and what the mindset of an aikidoka should be. I get the feeling from reading this book, that these were originally orations, from O-Sensei and not written down by the founder. I don't get the feeling that these were never meant to be collected together. It's not that they contradict each other, or are mutually exclusive, they just seem disjointed. None of this distracts from the wisdom and philosophy that is presented. Especially if it is directly from the source, it has a bit more gravitas to it.
5. Aikido Ground Fighting and Atemi, the Thunder and Lightning of Aikido, and Following the Martial Path by Ken Jeremiah and Walter G. Von Krenner
For this last one, I'm choosing a trilogy of books. These are written about the experiences of Walter G. von Krenner, a student of O-Sensei, and his time in aikido. The books themselves are intended for a little more advanced audience, so I'm going to include them for that reason. The others are more introductory, but by no means simple. The first two of these books are about how aikido could be used as a martial art of self defense, and how the techniques already being taught can be applied on the ground to become groundfighting. The second shows how and why atemi is applied in aikido in order to make it an effect civilian self-defense art. The third, is about the more spiritual side of martial studies, especially aikido.
I hope these give you a list that can add to your reading list for aikido.