When can I read "Beginnings"?

I ordered it when I was almost done with Wizard and Glass, and I'm currently about a third through Wolves of Calla when it arrived.

Will it spoil anything major if I pause to read the graphic novel? I know why Cuthbert and Alain aren't with Roland, but it hasn't been revealed about Rhea and Sheemie yet. Is it better to wait and read it from the books, or does it matter? Thank you.