Here's why:
* Metaphor: A figure of speech that compares two unlike things directly, without using words like "like" or "as." In this case, reading is being compared to a journey.
* Idiom: A phrase that has a meaning different from the literal meaning of the words. This phrase doesn't have a unique, established meaning beyond the literal interpretation of the words.
The phrase uses the concept of a journey to convey the exciting and engaging nature of reading.