Thanks for your interest in Graph Databases. I've contacted the author regarding your question, and here is his reply:
"START n=node:user(name='Michael') refers to a node index (as opposed to a relationship index) called 'user'. With this index, we're looking for nodes with a 'name' property whose value is 'Michael'. Any nodes that we find in the index, we then assign to the 'n' identifier, so that we can refer to them later in the query.
Unfortunately, we don't explain indexes in any great detail in the book. This is a bug - somehow the text dropped out at some point in the writing process, and I never caught the omission when I was pulling it all together. I wrote a short blog post a few months ago to help people who wanted to code along with the examples in the book:
Hope this helps and have a great day!
Customer Service Representative