I think it's safe to say we'd read a teacup and love it if J. K. published it and named it "Hogwarts in a Teacup" or "How to be a Wizard in 2 Sips". We would definitely all agree we'd swallow it whole and smile afterwards, asking for more. However, it's good to know that J. K. never offers us bad reading material (well, except for Cursed Child, which as a read is meeeeeeeeh but as a play I bet is marvelous) 
 Now this book is cute. I finished it in less than an hour. I liked it cause 1) J. K. wrote it 2) It's nice to see into her mind and the process of writing the HP series 3) Well, I'm a sucker for Magical History. 
 Still, I gave it a 3 for the reason that all I read in this "book" I can find or would have preferred to find on Pottermore. I mean, there are much more elaborate essays on there than in this book. 
 I especially liked the Hat story, as the sorting ceremony has become iconic. Just think that had J.K. gone with her first impulse, we'd be talking about a sorting machine, akin to Dr. Who or something. 
 All in all, you should read it if you're a true fan. And when you're done, log back on to Pottermore and continue reading similar (if not the same) stuff on there.