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.

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 also natural to say テスト前に3連休があるので勉強日和だ (although some may argue this is not a traditional usage). Either way, 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.