adverb
indeed; really; truly; very much
Emphatic adverb expressing strong affirmation or degree. Often used in Kansai dialect, but also found in standard Japanese with a slightly old-fashioned or literary feel.
本に、あなたの言う通りだ。
Indeed, you are right.
本にお世話になりました。
I am truly indebted to you.
本当に is the standard, neutral way to say 'really' or 'truly'. 本に is more emphatic and often dialectal or literary.
実に also means 'indeed' or 'truly', but is more formal and often used in written language or speeches.
Derived from 本 (ほん, 'true') + adverbial particle に. The exact historical development is uncertain, but it functions as an emphatic adverb.