There are various ways of searching for Books:

Global search allows you to return results from compatible apps on your phone without starting the app
Typing into the box returns a set of matching Books
All global results are always in Title order:

Pressing one of the displayed Books will display the Edit Details Screen for that Book

Allows the user to type a search phrase, which will then return all matching Books
See the Search Screen for more details

Allows the user to search for Books by Title/Series/Author
It matches the found results against your shelves
See the Find & Add Screen for more details

First decide which field to search for:

You will be presented with a choice of websites to search:

Your browser will start - searching the chosen website for your chosen field

You can scan an ISBN to start a search for a Book
If searching the web, you will be presented with a choice of websites to search:

Then scan a barcode:

If searching the web, your browser will start - searching the chosen website,
otherwise a Book search will be started using the Search Books Screen.

You can type some text to start a search for a Book
If searching the web, you will be presented with a choice of websites to search:

Then type what to search for:

If searching the web, your browser will start - searching the chosen website,
otherwise a Book search will be started using the Search Books Screen.