Went on a half-day fungi foraging course recently and asked what book to get. The answer was "several". That was more geared towards eating, though, and the rationale was that you should check multiple references to maximise the chance of positive ID and minimise the chance of a false positive. for the same reason, the Mushroom Lady also said to get books that have illustrations and books that have photos.
I ended up getting the Collins fungi Guide mentioned by Macaroon, and the River Cottage Handbook. The Collins is excellent - comprehensive range of species and great illustrations. The text entries are a bit terse (uses abbreviations for some bits), but cover the main info. The River Cottage one is specifically about eating, but also includes some poisonous ones that look similar to some edibles. Far less species covered, tending to focus on what is actually tasty. Text is good, easier to read, and it has good photos. Both have a good introductory section, with the Collins maybe a bit more comprehensive.
Agreed that the Collins is a lump of a thing at about A5 size and around 1.5" thick - a bit big for a jacket pocket, but certainly satchel-compliant. I got mine from Waterstones, bought online for about 14 quid, then picked it up from the local shop a few days later. That said, if you can't wait, and you're standing in a shop with it in your grubby mitts, it's well worth the 20 quid normal price to walk out the door with it.