The eBooks on the Library Catalogue come from different sources. The majority come from ProQuest Ebook Central, a large eBook library, but some come from another library called eBook Collection (EBSCOhost), and a few are linked through to a book or document on the Internet. If any of the books you access through the catalogue have a DOI, use this when referencing. Otherwise, reference as follows:
This example from ProQuest eBook Central looks like this on the catalogue:
Clicking on the link takes you through to the following page:

Once you have opened the eBook, you will see in the top left hand corner is the ProQuest eBook Central logo indicating that this is the collection this eBook comes from. Because this is only accessible by Wintec staff and students by logging in, and is not available to the general public, a URL is not given and the reference would be done as follows:
Gulanick, M., & Myers, J. L. (2014). Nursing care plans: Diagnoses, interventions, and outcomes (8th ed.). Retrieved from ProQuest eBook Central database.
Similarly, if it comes from the EBSCOhost eBook collection (in which case you wouldn’t see the ProQuest logo and it would say EBSCO on the catalogue or eBook Collection (EBSCOhost) when you click on the title in OneSearch), you would follow the same format but change the database name as follows:
Eklund, R. C., & Tenenbaum, G. (2007). Handbook of Sport Psychology. Retrieved from EBSCOhost eBook Collection.
Books linked to the Internet: Because these can be accessed by anybody, they are treated like any other web document, for example the Wintec Annual Reports entry on the catalogue looks as follows:

On clicking on the link, you are taken through to the Internet:

The reference would look like this:
Waikato Institute of Technology. (2014). Annual report. Retrieved from http://www.wintec.ac.nz/about/who/Pages/index.aspx
These ebooks are freely available on the Internet, so anyone can access them. They would be referenced as the example below:
Schneider, Z., & Whitehead, D. (2013). Nursing and midwifery research: Methods and appraisal for evidence-based practice. Retrieved from https://books.google.co.nz/books?id=siizbjoIMmUC&source=gbs_similarbooks