There are six books in the series, although the inner dust cover hints at a seventh X-Men book, which I have not seen. They're written in simple storybook style with short, easy to read sentences and lots of large and colorful art. They hew pretty closely to the old school origin stories of Marvel's flagship characters (much more so than the current comics!), which of course is always a good thing.
I of course bought the books for the illustrations. I love the art of vintage Little Golden Books and like to study it to see how it was made. The illustrations here are bright and colorful and have the look and feel of old Little Golden Books.
Each of the books was illustrated by a different artist of team of artists, but they all share a uniform look. I like the art in them all, but of the six I think my favorite is the Spider-Man book. The color and lighting effects in it are very well done. The Spider-Man art is somewhat reminiscent of the work of the Hildebrandt brothers (or the Brothers Hildebrandt, as they preferred to be called).
If you've got a child who's all amped up for superheroes after seeing The Avengers for the third time, these books would make a perfect introduction to the Marvel Universe. Much more so than the actual comics these days, sad to say. I don't know what's going on in the comic industry, but in the past decade or so comic books have mutated from fun adventure stories suitable for kids to dark, grim and disturbing tales made for basement-dwelling 40 year olds. There's no way I'd let a kid of mine read a comic these days unless I leafed through it first. It's gotten that bad. These books are a fun and worthy substitute.
So far the only place I've seen these is at Target, but then again I haven't been anywhere else for a while (I'm quite busy with my duties as President and CEO of Bob Canada's Blogworld) so it's entirely possible other stores may carry them.