There are only two really dangerous funghi in the UK - the Panther Cap (Amanita Pantheris)and the Death Cap (Amanita Muscaria).

The worst anything else will do to you is give you the worst upset stomach (at both ends) that you have ever had in your life.

The so-called "Horse Mushroom" or "Yellow Stainer" will upset a lot of people and it looks very like the classic edible mushroom (but different stem ring and different gill colour, and of course the Yellow Stainer stains yellow when cut). Fortunately I seem to be among those to whom both are harmless.   

All out of books (except for the fact that I am still here).