I had a kitchenaid mixer on my registry and my husband's aunts/uncles went in on it for my shower and our wedding present. I use it all the time, so even with the high price tag it is useful and will last forever. I had added it not expecting anyone to get it, but I was thinking if we got enough cash I could use the fulfillment discount to buy it after. You get about 15-20% off of anything on your registry that doesn't get purchased, so many people add more expensive items with plans to purchase after.
As for the book, I think it's fine if and only if a) you are close to the couple, 2) you can include an honest note about the change the book has had on your life, and 3) you give it in addition to a regular wedding gift. If any of those are not true, then it's probably not a great idea.