they usually get the precedent info off computers these days (TBH this has been the case for about 30 years and was one of the first uses of computers).
criminal law especially has changed so much in the last 20 years that the books have become as obsolete as old computer books - I used to buy them cheaply to try and help squatters, ravers etc but found myself that loads had changed. Certain books which are about specialised aspects of the law which are still valid are valued second hand but go for high prices on Amazon (a relative of mine is a law graduate, and most useful textbooks are sold to the next generation of students when the first lot graduate).
I wouldn't underestimate the value of old books as a support for other items, I've got several here propping up two small loudspeakers so they are just the right height for when I am mixing music. Now you can buy stands for this purpose made of all manner of exotic materials but they are about £70 each, and even if I plan to use something more permanent the pile of books can be used to judge the correct height of everything for ergonomics..