In most cases, you can either download the whole book or just a chapter. However, for some e-book platforms, you can only download a page-range.
If you download the whole book, information as to the amount of time that the download will be available to you will feature in the drop-down options that you are given. This download period will depend upon the e-book platform, the e-book licence associated with the book and the number of copies held by the library.
In establishing the time period for which an item can be downloaded the Library has aimed to achieve a balance between the following needs: providing access to the title to as many users as possible at any one time, ensuring that content is meaningfully used for the full duration of the download, and providing individual access as generously as possible.
In some cases where access is only available to 1 user at a time, the Library has chosen not to enable downloads, in order to preserve access for other readers.
Information as to download periods on specific e-book platforms is available on the following help-pages.