In Ebook Central, you can download just what you need! Typically, you're enabled to download individual pages and chapters as well as the entire book. In some cases of limited availability of a book, your institution may limit the full book download.
When you download pages or chapters, you get a PDF file that doesn't expire. You can search the text, and copy passages for use in your research, along with a citation in the format of your choice. The number of pages you can download shares the same allowance as what you can print to PDF.
To download the entire book, you'll need Adobe Digital Editions on your laptop/desktop, tablet, or phone. Your library determines the download periods available to you, which may vary depending on the number of copies held by the library.