Only problem I ever had at the Copacabana crossing was Peruvians wanted proof of insurance. Since I was leaving Peru they gave up on me, but didn't seem to be looking for a bribe, anyhow. As long as there is no civil disturbance the crossing is a piece of cake; Desaguadero - a touch further south - is no problem, just a horrible place. You can also go around the top of Lake Titicaca. And, yeah, ditch the Lonely Planet - Cusco is a good place to find Footprint books.