The Book Sync plugin works great if you have a single device. It is much easier than having to tag the books and then remember to send them to the device. I use this plugin all the time.
I have been discussing with Kiwidude some possible enhancements to the plugin that might mean that you can be selective about which book goes to which device(s) when you have multiple devices. He now has several himself so has some incentive to add such support. However whether it will actually prove technically feasible and (assuming it is) when he will find time on his extensive TODO list I am not sure.