The library may use portions of its Ebook Central ebooks in its customary ILL program under Section 108(g) of the Copyright Act.  ProQuest-specific guidelines regarding Inter-Library Loans (ILL) can be found in the ProQuest Terms and Conditions.  The relevant section is "Exhibit A", Section 1.e.i.iii, which is:
  i.    Interlibrary Loan (ILL). Library Customer may loan digital or print copies of materials retrieved from the Service to other libraries, provided that (i) loans are not done in a manner or magnitude that would replace the receiving library’s own subscription to the Service or purchase of the underlying work (e.g., newspaper, magazine, book), (ii) Customer complies with any special terms governing specific content or licensors as described in this Agreement, (iii) with respect to e-books, copying is limited to small portions of a book, and (iv) Customer complies with all laws and regulations regarding ILL.
However, given the high costs traditionally surrounding administration of ILL programs and the need for libraries to track DRM, we recommend the borrowing library use an Ebook Central Short-Term Loan (STL) as an alternative; this allows libraries to borrow entire ebooks directly from Ebook Central at a fraction of the cost of buying.

STL provide patron-specific short-term access to Ebook Central books.  STL allow the patron to view the entire book online, copy text, print pages, download pages of the book, and even full-download the entire book for the duration of the STL.