I used this book on a 9 week trip to Cameroon, starting in Douala and travelling overland to Mora in the Extreme North, so I put the majority of the information to the test. I found much of the information out of date.
A much better resource was a printed copy of the 40 or so pages on Cameroon that Lonely Planet do on their website as a chapter to buy from their Africa book as a PDF. It takes up less space, has better information and considerably less guff.
By all means buy this book for the pretty pictures, otherwise, the rest is best regarded as door stop/ fly swatter/ improvised fan, as You will rapidly discover it to be useless in comparison to the Lonely planet printout in a plastic wallet at the top of your bag, which will become a prized possession.