1. I guess you mix up "idioms" with "phrasal verbs".
Under idioms we usually understand something like "every cloud has a silver lining", "let's sleeping dog lie", "take something with a grain of salt". There's a huge range of idioms in English and they are utterly helpful in certain situations when English words simply cannot describe something.
As far as "phrasal verbs" are concerned, they are a combination of a verb and preposition, or a verb and an adverb, or a verb with both an adverb and a preposition. Sometimes a phrasal verb has a meaning which is absolutely different from the original verb. On the other hand, from time to time meanings of some phrasal verbs are really easy to guess. As for me, I do try my best to memorize them, however I still do not use many of them in my everyday speech. I mean they're like stuck in my "passive vocabulary".
2. I'm a student and I'm learning languages for the most part, so books containing many phrasal verbs, idioms, natural expressions etc. are extremely useful. That's why I rush to the biggest bookstore in Moscow if I feel that I badly need a new book of this kind .
I'd highly recommend you series of books called "English Phrasal Verbs in Use" by Cambridge Press. They're divided into several groups based on a learner's language level. There are "essential", "intermediate" and "advanced" stages or levels whatever it's called. Moreover, you can find not only books about phrasal verbs but also about idioms "English Idioms in Use", or collocations "English Collocations in Use" and so on and so forth. Certainly, there are many other books besides these ones but it's just my humble opinion.
I hope that helps.
Student or Learner