The methods used would vary by the value and type of paper and binding of the book.
If the books are valuable or irreplaceable the only real option is a professional book restorer.
For a relatively inexpensive mass market book, whether hard or soft bound, I might try the baking soda, and lying them open in a protected area during sunny dry days. Killing the mildew is the key.
I avoid febreeze because I would be leery of the chemicals damaging the paper. It also makes my mucous membranes bleed, so it is not on my list of favorites anyway.
Removing mildew and mildew/mold from any organic, fragile substance is difficult as you already know. Washing with strong bleach solution and drying in the bright sunlight is the primary remedy for mold, and as good as it is, it does not always work, and is certainly not advisable for every surface.
Good Luck on saving the books!