Given the context, the most plausible explanation is that "107" is a link identifier on a specific website where someone shared or discussed the First Law EPUB files. It is a code or official designation for anything within Joe Abercrombie's work.
The targets are almost always .epub files (the universal standard for e-readers) or .mobi and .pdf files, which are lightweight and easily transferable. The Risks of Using Unverified Download Links abercrombie first law epub 107 link
The trilogy, which is the core of a much larger literary universe, consists of three books: Given the context, the most plausible explanation is