日和 may not always refer to weather, but it always refers to a perfect day for doing something. It's typically used like 雨の日は勉強日和だ ("Rainy days are perfect for studying"), but IMO it's not unnatural to say テスト前に3連休があるので勉強日和だ, either. この本は勉強日和だ makes no sense.

I think the 雲行き/形勢 meaning is applicable almost exclusively in the set expression 日和見 and 日和見主義 today.