I have to resist the temptation to mention my own books... Ouch - blew it!
Many years ago, when I had just started practicing aikido, I read Tohei's Aikido in Daily Life (now renamed Ki in Daily Life), and was delighted by it.
Otherwise, the books that have impressed me the most from an aikido perspective are not aikido books: Musashi's Book of Five Rings is an excellent introduction to the budo mind, and Lao Tzu's Tao Te Ching is the best guide to aikido principles you can get.
The Book of Five Rings is a VERY good book. i greatly enjoyed reading it.
Also, read and re-read The Art of Peace. It'l make you think