I've been thinking about the guides recently and I sort of concluded that Peter Wermelinger has the right approach with The Funk Lexicon.
Although it's not strictly a price guide (Peter warns against them in fact!) the book, does give an indication of rarity, classifying records from A (very common) to H (extremely scarce). This approach is advantageous for various reasons: it's based on rarity rather than demand, it's not tied to one particular currency and therefore avoids the problem of fluctuation exchange rates. It's also immune to the impact of inflation. The big thing for me though is that it allows you to escape the tyranny of the dreaded fixed price.
Psychologically speaking, if you only have a rough guide to the rarity of a record, it makes it easier to pay a bit more when buying a record you really want. You're aware of how much you want to pay based on availability, rather than thinking you're paying £20/30 more than the 'guide price' Equally it's easier if selling to reduce the value to what someone's prepared to pay without feeling that you're losing that £20/30.
I'm aware that the Lexicon isn't perfect. Some of the estimates of rarity are a bit out and, of course, there's always the possibility of a quantity of a specific title turning up, which will clearly affect rarity/availability. On the whole though, I find it a more satisfactory contribution to my decisions when buying and selling funk 45s and I'm really glad that there are plenty of records in the Lexicon that are either missing or hopelessly under or over valued in John Manship and Tim Brown's guides.