According to "tips and notes" "If an adjective comes before a noun with a definite article (de or het) then it usually gets the ending "e". So in this case, het boek, with the adjective before means it gets the "e" ending. Hope that helps!
Students of German will recognize this to be similar to the word "Mühe". mühevoll -> mühelich -> mühelijke -> moehelijke -> moeilijke. So instead of saying schwierig, moeilijke could mean something which requires much effort!