I've been addressing the gaps in my reading of Appendix N. I recently finished reading "The Roaring Trumpet", the opening novella of The Mathematics of Magic
by L. Sprauge de Camp and Fletcher Pratt. Having never read it and knowing almost nothing about it it made for an enjoyable read, both in its tone (not goofy but not super-serious, reminding me of some of my more enjoyable gaming sessions) and in seeing just where a lot of ideas that found their way into D&D.
What'd this bring us? You've got your planar travel with different laws of reality, giants who'd be quite comfortable in Against the Giants
, and spell scrolls. I've posted a lengthier article
about this in my blog. It's rather fun to get the mental clicking as I see where various ideas in D&D made their appearances.