Overdue is an adjective means مُتأخر (muta'kher) U can translate the first sentence "our meeting has been long overdue" as لقد تأخر اجتماعنا كثيراً but it's better here to translate it to لقد طال انتظار اجتماعنا it refers that the speaker has waited this meeting for a long time. And the translation of this one "this book is overdue from the library" is هذا الكتاب تأخرَ من المكتبه.

overdue means متأخر