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".
travel.stackexchange.com/users/1452/SylvainD
I usually buy more than one guide book per trip.
Where one is weak another will be stronger.
I look up things I am likely to want to know.
For example will you rely on your guidebook for opening hours?
Then look up and see if they are there.
If you're going to use the internet for opening hours, then you don't care about details like that.
Do you want ideas and inspiration?
Are the maps readable?
If you want a little history, is there history or just a list of attractions?
Does flipping through the book make you want to go right now?
That's a good sign.
If it's a series, you're in a bookstore with a number of books to flip through, and there's a place you know quite well that is covered in the same series, look up things you know and see if the description matches your knowledge.
That will give you an idea of the quality of the series.
It might not carry over to other books in the series, but there's a good chance it will.
travel.stackexchange.com/users/46/Kate Gregory
In my opinion the best way is to search for online reviews.
Personally I like Amazon.
First there are the readers reviews, secondly Amazon has this section called "What Other Items Do Customers Buy After Viewing This Item?".
You can get some decent impressions with these to features of amazon.
Another possibility is to go to the local Library.
Here in Belgium, the library has a decent selection of travel guides.
Based on how frequently the book is borrowed, you can get an indication of its quality.
travel.stackexchange.com/users/None/
The first thing I check when comparing travel guides is when an edition was last updated.
I skim read the contents of cities/towns I'm visiting to see if I can catch anything particularly important that only a newer book will have.
I'm mainly bothered with transportation information as far as updated information is considered: the worst thing you want is to make your itinerary based on old data and then find it's impossible now.
Somewhat counterintuitively to the above point, if I don't find anything particular different in the transportation section I buy older editions of guides.
You can get these much cheaper on Amazon or second-hand bookstores and more often than note - apart from possibly changed phone numbers and price listings - most of the information is valid anyway
and I save a bundle on buying the guidebooks.
What I usually need is good background information about sights to see (always remains the same), locality maps (mostly remains the same, unless roads get renamed - which in some countries like India is a major pain in the butt), and even the best cafes / restaurants / hostels are typically ones that have been around for years.
Don't preclude that old editions of guidebooks are useless.
More often than not, I don't buy guidebooks.
I rely on Wikitravel.
What I do instead is buy proper maps in cities / towns I visit.
These give me much greater detail than guidebook maps and
since I like exploring a place mostly on foot, this greater map resolution helps me pinpoint places of interest better and plan my day.
Maps cost a fraction of the price of guidebooks, and in smaller towns you probably don't even need to pay for one - free maps handed out by the local tourism office should suffice.
travel.stackexchange.com/users/108/Ankur Banerjee
If interested in food it is essential to pick as fresh a book as possible since popular restaurants often tend to get worse over time (and the guide book reference may indeed add to such a trend).
I also want a thick one with lots of facts - not a story.
If I am interested in a place I can always delve deeper on the net or buy specific books about it.
Based on these criteria I found myself with the Rough Guides most of the time with the occasional Lonely Planet or Cadogan (after a quick check of my bookshelf.
If possible it is good to stick to the same publisher because they format all their books in the same way.
It makes it easier to start reading the new one.
travel.stackexchange.com/users/384/froderik
So I have decided which country I want to visit next and go down to the local travel book store. On the shelves they have over ten different travel guide books about the country.  How do I choose among them? Are there any tricks to find out how good they are?
How to choose a guide book for your next trip?
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<tips-and-tricks><guidebooks><shopping>
travel.stackexchange.com/questions/4582
travel.stackexchange.com/users/1317/Peter Hahndorf
Make sure the edition is quite recent, especially with food as the quality of popular restaurants can deteriorate over time. If the book is part of a series, look up things you know and see if the description matches your knowledge. Check a bookstore or library to compare several books. Obviously, the best book for you depends a lot on what you are looking for. Researching a travel book before buying is considered important. This can be done online, in the bookshop or at the library. If you have a special interest in food, a recent edition is a must as popular restaurant tend to go downhill over time.
A suitable travel guide depends what you are looking for. Some criteria include how well it covers a place you know, what its online reviews are like.  Check out books in a library before buying. Check when the last update was. This is especially important if you are interested in food as popular restaurants tend to go down hill over time.