The market fluctuates so fast these days that published guides are not particularly useful for commonly found instruments.
Where you get into trouble is if the bass is very rare... completed ebay won't help you there because you won't find anything comparable.
Even a published guide might mislead you, because rare stuff is not a commodity. The value of a rare/old/collectible instrument has absolutely everything to do with how badly the buyer wants it. One person may be willing to pay twice what another person wants to part with. So what's it worth? Price x or price 2x? Depends on who you're dealing with. No book can tell you that.
However, if you poke around enough, take your time and be observant you'll get a good feel for what the vintage market is like.