Two of the most popular guides to jazz are the "Penguin Guide to Jazz (on CD)" and the "All Music Guide to Jazz".
Out of the two, I like the All Music guide better. I like their album descriptions more, and they also supply other information. For each artist, they have a summary of his style and musical history. Like Penguin, they have a similar "rosetta stone" mark for truely outstanding albums but unlike Penguin, they also have marks for the best albums of that particular artist, along with a first purchase mark.
All Music also has some neat essays about the history of jazz, along with major artists and best selections (books and albums) for major jazz eras. All in all, I find this book to be "more accessible".