Obviously, the best book for you depends a lot on what you are looking for.
Many criteria are to be taken into account :
the place(s) you'll go to : some books will cover only the capital, other will cover the main cities and other will cover the countryside as well.
the duration of your trip (many books are explicitely for trips of a couple of days)
the kind of tourist you are :
- what are you really looking for (cultural visits, natural landscapes, relaxation)
- how much are you willing to spend on your trip (many books are for backbackers and might be irrelevant for a family trip) : the transportation, the places to sleep/drink/eat/visit and the can be really different
- how much cultural background you want to know : a few small sentences in the foreign language ; the whole history of the country ; a description of the culture you'll be living in. All those things can make a difference and might or might not be relevant to you
- how you want to use your guide : read the whole thing before the trip, open it at random once you are there, etc
- how you want to find the information in your guide : sorted by city/price/type of activity
- many other things
In any case, if you are in the shop, it's worth opening the different books to see if they really talk about what you are looking for and if it seems good to you (which is mostly of matter of personal preferences).
If you are front of your computer, you can still have a look at the readers reviews (on Amazon for instance) even if it might not be really accurate as one usually tries only one guide per trip/destination.
Finally, after a while, you might get used to a specific type of guide and feel more comfortable with it. As for me, I'm really used to the "Guides du Routard" (French collection) and I'm not really fond of the "Lonely Planet".