If you are that level, you should also know the Ritual Of Rain. Unless you are starved for Mana, you still can learn the new spell straight away even if it's not raining.
Yes, you are right, but ...
BTW, that's why I've indicated a bug about activating this spell in the new Laboratory to boobsgames (he has fixed it in the last release). I thought I could cast this spell to learn the new Thunder spell.
Unfortunately, it seems (but I'm not quite sure) it must rain "naturally" when you want to learn the spell by reading the book (i.e. at the end of your reading - during the night). Casting the Ritual Of Rain in the evening before reading the book is too soon, and casting it during the night after reading it is too late.