"The Norse myths ... [are] of a people who did not entirely trust or even like their gods, although they respected and feared them." (Gaiman's introduction.)
Gaiman has assembled a dozen or so stories from Norse mythology, using mostly the Poetic Edda and the Prose Edda and their often varying versions of tales to construct a cohesive set of stories in plain modern English. He writes of a god who had turned himself into an eagle and was "flying so fast he made a boom as he passed through the air" (the quote is from memory and possibly not exact, but the substance is there) - so Gaiman, without using modern colloquialisms like "sonic boom," is implying them - but making something entirely readable even if you're somehow not familiar with that concept.
The Norse gods don't come off terribly well in their own mythology: I was probably around 12 the last time I read an interpretation of these stories, and back then they were just exciting adventures to me. As an adult, it's clear that this is a bunch of petulant, childish beings with utterly terrifying powers who should be avoided at all costs. Loki in particular, of course, but unlike the God of Christianity, these gods kill for their own amusement and really don't give a damn about the humans on Midgard.
If you're not familiar with the Norse myths, you should read this yesterday. If you are familiar with the Norse myths, you should read it today. If nothing else, this is a great time to see a more accurate representation of the characters than that shown by the Marvel Cinematic Universe. Gaiman brings all the gods and giants and dwarves to life beautifully. A great read.