It's true what they say; the natives are friendlier when you make an effort!
I have purchased a variety of language guides for a number of languages (Czech, Italian, French, German), and I have always found the LonelyPlanet phrasebooks to be the best. Their size makes them very portable and discrete, they are very well organized, and have good, easy to use 2-way dictionaries for when you're in a pinch. They also don't take themselves too seriously, and include sometimes laughable - though potentially very useful - sections on things like clubbing, and "intimate" times. The guides claim to be "more essential than socks (and lighter on the nose)." I think this sums them up nicely.
In particular, I found that the Czech language guide did a fantastic job hand-holding the reader through the admittedly arduous task of learning to pronounce all the different consonant- and vowel sounds with their plethora of accents. Be sure to start there.
In short, this is an indispensable guide, and an absolute MUST for anyone traveling to the Czech Republic.
P.S. One word of warning: no book can really teach English speakers the infamous [r-with-upside-down-^] or "rzh" sound - as in the Czech composer Dvo['rzh]ak - since the sound does not exist in our language. You'll see what I mean, no doubt. I suggest searching online for an audio clip of this consonant. The word [rzh]eka, for river, is an excellent example, if you can find it. Happy traveling!