They have their uses for the writer, I own several of them myself, but their origins for names are not exactly of the highest quality. And even if they've gotten the location right, they don't give you any clues about the era. Name a character Holly in medieval England, or even a pseudo-medieval pseudo-England, and the name's being English and a dollar will buy you a cup of coffee. (Nickname, maybe, but you will need an explanation for that.) Not, of course, that your troubles are over when you have names for the right era, either the one you put your story in or the one you want to suggest an analogue to by your name choices. Some, perfectly authentic, don't sound old. Like, say, Tiffany. Sometimes they don't even convince me; "Alice" is a fine old name but had Victorian connotations to me -- well, up until I used a different spelling and set out with my heroine "Alys". And authentic combinations of old stand-bys and names that have fallen out of use since that era may have readers complaining about the mix of real and made-up names. (sigh.)
And there's the other danger of baby names books: they list meanings. Sometimes they are right and sometimes they are wrong, but in either case there's the temptation to give the characters a Meaningful Name. Except that unless it's seriously iconic, like Rose or Faith (in which case the characters had better comment ont it), odds are that your readers will not know the meaning. So it can, at most, be a fillip that selected readers, those who know the meaning, can see; the story can't really use it. Probably just as well. Meaningful Names have a tendency to be comic, since they are such wild coincidences.