To paraphrase RS, why skimp on a $20 guide book (or an updated one) when you are spending thousands of dollars on your trip? It could make the difference in terms of transportation info, restaurant and hotel listings, contact info for people and places, operating hours of museums and other sites, etc. Admittedly, maps and recommended main sites are probably not going to change much in 3 years, but a lot of other things could. I would recommend always traveling with the most current guidebook (and a cell phone with data service when you are away from wifi).