Re: Aikido Books
I don't think anyone mentioned Saito Sensei's books. Unless you have a lot of cash, his original "Traditional Aikido" series may be out of reach. But over at aikido journal they have the entire second series, Takemusi Aikido. Incredible technical manuals.
I've only heard Imaizumi Sensei speak highly of two of his teachers (although he says little so I'm sure there are many others he respects), Saito and of course Tohei. I received a copy of "This is Aikido" as a wedding present and read it about twice a year.
I also recommend becoming a member of aikido journal or purchasing the the DVD with all of the old magazines in it. It's one of the most complete collections of aikido interviews and articles in existence.