The Data Book goes with the Wilderness Press guide books. The idea is that the wilderness press guide books have a lot of wordy prose and every now and then there's a sentence about some point of interest and next to that sentence is in parenthesis, the elevation and the distance from that last point of interest. The Data Book went through all those points of interest and put them into a table and calculated the mileage between those points and added other information. Makes it way easier than trying to get that information out of the guide book.
Many of those points make no sense without the more descriptive prose in the guide book, though. But lots of people have used the Data Book without the guide book combined with maps and just guessed at those points of interest. Plus the Data Book has mileage information for off-trail water and lets you know if a point of interest has water or a road or whatever. Plus there are post office addresses which is handy. So yeah, I can see why it's not what you expected, but you might be able to use it anyway.
Most people are using Halfmile's maps and Yogi's guide these days. These days being this year compared to as recently as 2008.