I take an average of 4 books, Jon:
CCGB - which I too think is the most accurate
Spink - which, in my area at least, I think is too generous
British Coins Market Values, which is generally too low
Coin Yearbook - lower than Spink, but a little more generous than BCMV
For varieties, which are not covered well by the the three latter books (CCGB is better than most, but still not comprehensive), I use the values detailed in Davies (for silver), and Freeman (for Bronze) as relative values, and calculate using current values - so for instance, if we take the 1955 Scottish shilling as an example, Davies values Rev C at £3.50 in UNC, and Rev D at £1.50. These are 1982 prices, so the absolute numbers are meaningless - the relative numbers however, are still valid in my opinion, so I assume that the CCGB 2015 value of £8 is for the commoner variety and calculate the value of the scarcer Rev C accordingly: Rev C = 8 x (3.5/1.5)
I also make detailed records of what I have sold coins for, so those values get fed into the average of the 4 books too - this tends to come up with quite an accurate figure.