I bought a copy of Rough Guide to Croatia and then got frustrated with its lack of information on transportation and purchased a Lonely Planet Croatia book as well. Both turned out to be good for different things. The Lonely Planet book lacked some key maps including one of Plitvice National Park and Borik, the area near Zadar where most tourists stay. I have no idea how we would have found our hotel in Zadar (reviewed in both books) without the Rough Guide because Lonely Planet was no help.
I think the authors of both books are still learning about Croatia because they occasionally lacked information about places that the other had. Just not up to snuff like the Lonely Planets I've used to travel around Latin America.