megabook

is likely misspelled.

has no English definition.

As a noun megabook

is an extremely large book.

As nouns the difference between large and megabook

is that large is an old musical note, equal to two longas, four breves, or eight semibreves while megabook is an extremely large book.

As an adjective large

is of considerable or relatively great size or extent.

As nouns the difference between book and megabook

is that book is book while megabook is an extremely large book.