The Record Collector Rare Record Price Guide is the only real UK reference for pop and rock, but as mentioned by others above it is flawed it has some serious omissions, gets some values seriously wrong (both ways), fails to recognise the huge difference in value between a first press and subsequent releases on many titles, and is utterly hopeless on jazz (seriously undervalued 15 quid for a mint UK 1st press of KOB or ALS anyone? Ill take all youve got!). The other problem with the RCRRPG is that it is by design always quite seriously out of date, it is published every two years, and is the result of two years work (i.e. can be 2 years out of date at publication on some items). Record prices fluctuate quite a lot due to fashion, and whilst say a blue black mono Piper At The Gates Of Dawn will never go down in value, many other titles do (i.e. try shifting an Atomic Kitten collectable). It also never gives any idea at all on recent releases. I still find it an indispensable reference book, and with time you combine it with personal experience. This is the guide the dealers use, so it is by far the best starting point for any collector.
There is also a guide for classical pressings, the Rare Classical Record Price Guide 2004 published by Silverwood Publishing. This is very limited in scope, it only covers Decca SXL, SET, LXT, HMV ASD, ALP, Columbia SAX, SAN, 33CX, i.e. no DG, no RCA, no Philips, etc etc. It does however attempt to price every single release within all the series it covers in all issues / reissues. There are errors, and some of the pricing is a bit iffy, but again it is an incredibly useful guide.
In many ways eBay is the best price guide around, it gives a real indication of what will sink or swim at any given time, though there are huge inconsistencies in closing price. These are real prices that someone actually got for an item, this means far more than seeing a record at a silly price on a record shop wall that may well sit there for years unsold.
Tony.