I agree with this "like any other gift giving occasion. How much you spend is a function of your means and you closeness" but would add, that is it also a function of the means of the recipient.

I might give a very impractical cashmere baby blanket (albeit washable) or baby china to an affluent couple. For a couple with very limited means, I would choose something very practical ... even diapers (a lot of them clearly).

For any occasion where someone is registered, 80% or so of what I give is from the registry, with the other 20% a total surprise based on my own ideas (like gift giving of yore). And yes, a baby book, pref a board book, and not one of the top 10 most popular.