Well it's like anything else - you want to know the rules or guidelines first - then figure out when to break them!
The guitarist learns the scales first as a technician - then the more gifted players go on to create music as artists - regardless of the confines and constrictions any particular scale might seemingly impose.
Same thing here, various instruments have general frequency boundaries or ranges. Various frequency bands have more or less general pleasing level and dynamic ranges. Assuming a track or instrument is captured in an already well-balanced fashion (recording - the basic building block of the mix) then the single track has to be combined with other tracks creating the more complex mix. The Art of Mixing...stick around there's a lot of good posts to search through on this forum concerning just that!
I think in the book you mentioned though they are probably discussing various common frequency ranges of instruments - general ranges. There's really no way to discuss db levels of any particular instrument or track without knowing the intended arrangement & hearing the other tracks in the mix.
Here's a Freq Range Guidelines Table:
http://www.recordingwebsite.com/articles/eqprimer.php
You'll see this kind of stuff everywhere. The best thing to do is just sit down and listen as Glen said - listen to the various instruments on seperate tracks and changes that occur when you solo/mute them and also when you play with the level and/or EQ settings.
If you aren't sure you can record a well balanced sound yet (don't worry - it's the holy grail most of us try to shoot for) then go to one of the sites hosting mixdown competitions on the internet. At various times you can find raw tracks from Madonna, Peter Gabriel, etc to play with to get your studio calibrated a bit. You have to be able to hear the tracks as well as your changes to be able to mix it well. In other words if you're increasing a bass track 3db and you don't hear that change the balance of the mix then you've got to take a look at your monitoring situation, etc...
2cents
Good Luck!