what is meant by this second last term?
It is a term for a novel written as a series of documents - usually letters, diary entries, newspaper clippings and other documents.
Epistolary novel
An epistolary novel is a novel written as a series of documents. The usual form is letters, although diary entries, newspaper clippings and other documents are sometimes used. 
who wrote the perks of being a wallflower Stephen Chbosky is it a novel? Yes, it is a young adult coming-of-age epistolary novel.
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Epistolary novel [SEP] Introduction
An epistolary novel is a novel written as a series of documents. The usual form is letters, although diary entries, newspaper clippings and other documents are sometimes used. Recently, electronic "documents" such as recordings and radio, blogs, and e-mails have also come into use. The word "epistolary" is derived from Latin from the Greek word ἐπιστολή "epistolē", meaning a letter (see epistle). The epistolary form can add greater realism to a story, because it mimics the workings of real life. It is thus able to demonstrate differing points of view without recourse to the device of an omniscient narrator. An important strategic device in the epistolary novel for creating the impression of authenticity of the letters is the fictional editor.