When we click enter it simply does get request by navigating to URL. Try below code.
Sub SearchBook()
Dim URL As String
URL = "http://www.nook.com/gb/store/books"
Dim bookname As String
bookname = "visions trips and crowded rooms"
Dim IE As Object
Set IE = CreateObject("InternetExplorer.Application")
IE.navigate URL
IE.Visible = True
Application.Wait Now + TimeSerial(0, 0, 5)
MsgBox "Site will now search for book"
IE.navigate "http://www.nook.com/gb/search?utf8=%E2%9C%93&%5Bs%5Dkeyword=" & bookname
End Sub