Books can be a good source of reference on techniques, but never ever replace actual practice with a good teacher. It all depends on how you use it. But that goes for any regular Aikido lesson
Do not just read any Aikido book out there when starting Aikido. Because of the many different styles/angles you might get more confused than 'helped'. Ask your teacher.
Books can help to understand the philosophy, the ideas/concepts of Aikido. A book allows you to (re-)read when you want to, in your own time. A teacher may find himself running short of time to elaborate on the non technical parts of Aikido during classes. People come to practise, and can easily study nontechnical aspects at home. Perhaps a next lesson one will have questions the teacher can respond to. Seminars could be a good tool for a teacher to focus a bit more on nontechnical aspects of Aikido.
I do however think that the teacher should at least explain why he thinks it is important to read the books he promotes.