In Part 1 of “Japanese Swordsmanship” (1982) Gordon Warner and Donn F. Draeger provide a good overview of the development of the Japanese sword within the culture of the samurai. Part 2 deals thoroughly with Iaido. While today there are videos that show the techniques better than any book can, this text remains a useful guide to the topic. Even an Aikidoist who does not practise Iaido can learn a great deal from this fine exposition.
The following quotation about the (idealized?) meaning of Iaido can equally apply to Aikido:
Modern Iaido is a reflection of the morals of the classical warrior. To train properly is to release such morality among modern men. The moral self appears only when one faces adversity over a sufficiently long period of time. Mere theorizing will not produce the elements necessary for the release of the moral self. Such training (shugyoh) requires the exponent to bear hardships calmly, to make desperate efforts to persist in training, and to make correct judgments in the numerous crises that he encounters in his training…Iaido is an art that enables the exponent to contrast the concept of “life” with “a life worth living”. [p. 100]