It depends upon species, but I have eaten quite a few of the garden variety fern curled tips or fiddleheads (unfurled fronds) raw. They taste somewhat pleasant with a bitter or dry texture but a mild taste. Ferns do contain toxins, possibly carcinogens, and they uptake heavy metals (including arsenic) from the soil. So eat sparingly, and it is probably best to boil first and throw out the water to remove some bitterness and toxins, although the half-dozen or so at a time that I have eaten raw did not make me sick. Take no more than three per plant, as harvesting more can kill the plant.
I suggest reading all you can about the species you intend to consider for food. Find out if there are any poisonous members in that particular family and learn to recognize them, and also any look-alikes from other families, and narrow it down from there. You should be able to at least identify any plant you are interested in down to family. Knowing this can tell you many things including whether or not it is a safe family (some plant families have no poisonous members), and from there, identify to species. I suggest The Jepson Manual (only for California) or other scholarly regional field manual (these books are hefty, and are thus not field guides per se, but learn to identify and be certain before you need to rely on any plants you intend to eat), a botanical dictionary (usually not necessary if your manual is worthwhile as it will have an extensive glossary with drawings), a high quality 10x folding handlens (attach a neck lanyard), and a small transparent ruler with metric, as must tools for anyone interested in proper plant identification.
Finally, plant identification is not always an easy task, and like Chris said, there are many books out there written by laymen. These "guides" will give you a false sense of security if you rely on their minimal and amateur descriptions and drawings/photos. I suggest taking an intro field botany class just to get an idea of what goes into being able to reliably identify a plant growing wild.