According to the International Publishers Association and the World Intellectual Property Organization's publication "The Global Publishing Industry in 2018", there were 3,485,222 International Standard Book Numbers (ISBNs) registered in the United States in the year 2018.
Yeah. Nearly 3.5 million book numbers issued in one year alone.
The Library and Book Trade Almanac for 2019 said there were 194,679 books published in all the places they tracked for the year 2018. Which is more than an order of magnitude smaller than the issued ISBNs, but still, no library staffer that I know of, not even Librarian Georg, has been able to read 200,000 books in a year on work time. It would require reading and comprehension at approximately the speed of Time Lord to try and get through everything that had been published in any given year.
So, it's pretty clear at this point, that no one library worker is going to be able to read all the things they're supposed to have read and know everything about to do recommendations over.
So, since we can't know everything, our goal has to shift from finding the perfect book for someone (which only works if you have infinite time and resources to carefully consider everything and find the item that's just right) to finding the next book for someone. Sometimes you're going to hit one out of the park, sometimes you're going to bomb horribly. Most of the time, though, even if you fail or you don't hit the mark as well as you would like, you're not going to alienate the person you're giving recommendations to so much that they refuse to listen to you ever again. Keep your wins in mind to try and build on them, don't get personally attached to losses, but instead see them as more information for you to refine your work into something better so that you're more likely to get a win for the next time.
Now that we've set our sights on reasonable goals, it's time to examine the wealth of information available to the library professional to help them recommend a book to someone, even if they haven't read it all the way through, or at all.