Re: aikido is the art
Sphere is truly a beautiful book, but as far as an aikido textbook, it's falls somewhat short. The authors did a bang-up job with what they knew, but they made some assumptions, based on incomplete research, and also apparently simply overlooked much. The authors' 'Secrets of the Samurai' was as beautifully illustrated, but woefully much worse in terms of their research and journalism.
Better intro-level books on aikido might be the works by Ueshiba Kisshomaru, augmented by, depending on your affiliation, some of the Tohei or Shioda books.