Please don't double post (I saw your post in foundation design, this doesn't belong there anyway).
I can recommend Dynamics of Structures Theory and Application by Chopra
Furthermore you have code requirements and simplifications, and a wealth of resources about those are available from SEAOC, ASCE, and SK Ghosh.
For horizontal distribution you have three basic assumptions, Rigid Diaphragm, Flexible Diaphragm and semi-rigid diaphragm. If you understand well the first two concepts then the third can be understood as well.