There is no shortcut as such, but simply typing the following
/authors:~^J
Then press enter
will restrict your calibre book list to only books that have an author whose name starts with J
You can then sue any hotkey program like auto hotkey to create your own shortcut for this.
Similarly, if you want to look for authors whose last name starts with J
/author_sort:~^J
The leading slash focuses the search box in calibre.
Although, since you say you want to search for books by Henry James, just type
/authors:James
or more precise
/authors:"=Henry James"
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