As salām ʿalaykum wa raḥmatullāh wa barakātuh, Ustadh. Knowledge one has without acting upon it can testify against them. Does this apply to books one buys but does not read immediately? Is it blameworthy to keep buying books with the niyyah to study them, without finishing the ones already owned?

No, this is not blameworthy, and this is not what is meant. What is meant is the knowledge that you have actually acquired and you don't act upon it, or you violate it. As for buying books, this is something that is good if you can afford them. Sometimes you don't even know; maybe somebody inherits them, or someone in your family when they grow up might take some of these books and study them, and you would get rewarded for that as well.