American Libraries offers a brief guide for libraries on purchasing ebooks. The guide describes how buying (or subscribing to) ebooks differs from that of traditional paper books. The process is much more complex.
From the article:
Transitioning to ebook purchases in libraries offers many opportunities and challenges. These challenges, however, are not insurmountable. New business models continue to emerge. Changes and improvements are occurring in the industry every day. These changes will continue as publishers, libraries, and vendors experiment with the growing market of ebooks. The most important thing that librarians must do in this changing environment is to articulate clear ebook purchasing goals. With these goals in mind, libraries need to find the content they desire, seek the best price possible, determine sustainable business models, analyze license agreements, and evaluate vendors to effectively purchase ebooks.
Read the article A Guide to Ebook Purchasing.