The Bible

 This is Glass's Guide - known as 'The Book'  or  'The Bible'.  No self-respecting car salesman would ever be without it.  At car auctions it was essential.
It lists all models available with details of engine size, equipment etc. etc. - but most importantly the TRADE PRICE of the car.  The starting point of any sale (or purchase).
 First published in 1933 the guide was updated every month and was only available to bona fide traders.  It grew and grew (this 1997 version has 976 pages) - and gradually became too unwieldily to consult easily.
The last paper version was published in August 2016 - now it's completely digital - via 'phone, tablet & computer.  If you have the registration number you can get an immediate valuation of that specific car using some very complex and clever programming.
Looking within this 1997 edition I see that a basic Ford Fiesta cost £8,900 and a 12 month old model would be worth £6,800.
A 2021 Fiesta costs £17,000 and after 12 months would be worth £14,800.  They are, of course, completely different cars as you can see...