TITLE: Introductory reading on the Scholz reflection principle?
QUESTION [10 upvotes]: The Scholz reflection principle says, among other things, that if $D < 0$ is a negative fundamental discriminant, not $-3$, then the 3-ranks of the class group of $\mathbb{Q}(\sqrt{D})$ is either equal to that of $\mathbb{Q}(\sqrt{-3D})$, or one larger.
Does anyone know of (and recommend) any introductory reading on this fact? Why it is true, what context to view it in, etc.? Googling reveals some highbrow perspectives on it, some interesting applications, and citations to Scholz's 1932 article (which I'm having difficulty accessing for the moment). All of this is interesting, but there doesn't seem to be any obvious place to begin.
Thank you!

REPLY [7 votes]: Hi Frank,
There are two places that I remember reading, and enjoying, when learning about the reflection principle:
i) Washington's book on cyclotomic fields section 10.2.(This book is just so great, so in case you don't own a copy this might be a good excuse to buy it.) 
ii) Reflection principles and bounds for class group torsion By Ellenberg and Venkatesh--This is a VERY cool paper, but in case you are short on time you just need to read Lemmas 4 and 5. 
Also some of the answers to this question Explicit map for Scholz reflection principle might help a bit.