To make one pile you would only need 10 books. The question doesn't say how many piles. It only says that you can't have more than 20. It also says or appears to say that you can't have a pile with less than 10 books. And "or" means you have a choice. So if, for example, you had 15 books you couldn't make a pile of 10 and a pile of 5, you would have to have a pile of 15; but if you had 20 books you could have two piles of 10 or one pile of 20.
It looks like the minimum would be 10 because you couldn't have less than 10 to make a pile.
Now, let's look at another interpretation of the question. The LCM of 10, 15, 20 (5*2, 5*3, 5*4) is the LCM of 2, 3, 4 multiplied by the HCF 5. The LCM of 2, 3, 4 is 12 so the LCM of 10, 15, 20 is 5*12=60. 60 books can be piled in different ways: 6 piles of 10, 4 piles of 15, or 3 piles of 20. Or they could be piled as 2 piles of 10 and 2 piles of 20; or 3 piles of 10 and 2 piles of 15, etc. The question doesn't say if the piles have to be all the same size. The minimum number of books that can be piled in piles of 5, 15 or 20 books a pile is therefore 60.
So it depends on how you interpret the question. If you have been working with HCF and LCM recently it's likely that the question has been given to you to apply your learning to a problem. So the context of the question may be significant and may guide you to the intended interpretation. If you have been learning about LCM and HCF then go for 60 as the answer.