One of the benefits of eBooks is the ability to search within the text.

Two of the most common eBook platforms have the search function prominently displayed on the left hand side of their interface:

ProQuest Ebook Central:

This is immediately displayed on the left.

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VLe Books:

On this platform you have to click on the magnifying glass icon to bring up the search screen.

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Almost all platforms have this function, but what it looks like varies between supplier.