The study of Fungi is one of those subjects where, once you get into it, you realize the more your learn the more there is to know. There are estimated to be between 2.2 to 3.8 million species of fungi in the world with only around 150,000 (6%) actually identified. My point being, the subject is huge!
I no longer try and carry a handbook for mushrooms - there are just too many of them for a small book to cover adequately. My go-to references are 'Edible Mushrooms' by Geoff Dann who sometimes contributes on this forum (over 300 species covered) and 'Mushrooms' by Roger Philips.
I have decided a long time ago I will not be able to identify every fungus I see (I attended a four-day course where the university lecturer couldn't identify some species we found) so I concentrate on searching out edible species that I am confident I can identify. I study carefully what I am looking for, go out, collect samples, return and make a 100% identification (otherwise I discard that species) and return to collect if I want to. The small handbooks just do not give enough information to be 100% positive in many circumstances.
Just my opinion, I know other people are happy using the small field guides; I've just seen too many very close look-alikes to feel confident with them.
I started out using Mushrooming Without Fear by Alexander Schwab (it has a terrible cover, but don’t let that put you off) which is a really simple, helpful guide. The original book didn’t cover mushrooms with gills, but he updated the guide and published Mushrooming With Confidence which I have also used extensively.
The best updated reference I have is Geoff Dan’s book, Edible Mushrooms which is excellent.
The best advice I was ever given is make sure you compare mushrooms across multiple books / field guides.