the price guides will give you a very accurate relative value of a piece. if your bridge is selling for 17% less than book, you can probably count on other new piece listings in the same -17% range. of course the larger the sample, the more accurate your estimate will be.
as far as older post and prewar, there are far too many variables to make an accurate and usable guide. not only are there quite a few more conditions than just "good" and "excellent", but there are also longer term market trends. with more collections being put up for sale in recent years, the general trend is that more common pieces are seeing a much heavier discount of the book value than more desirable pieces. but the bottom line is that if you see a book price of $100 on Car A and $300 on Car B and they are both fairly common, regardless of the actual price being realized for Car A, you can bet that the average realized price for Car B will be ~3x as much.
