hit the books vs mug up

hit the books and mug up both are verbs.

Word	Noun	Adjective	Verb	Adverb
hit the books	No	No	Yes	No
mug up	No	No	Yes	No

As verbs, mug up is a hyponym of hit the books; that is, mug up is a word with a more specific, narrower meaning than hit the books:

Other hyponyms of hit the books include con, learn, memorise, memorize, bone, bone up, cram, drum, get up, grind away, swot, swot up.

hit the books (verb)	mug up (verb)
learn by reading books	study intensively, as before an exam