It depends on the reason. Is it a book nobody much could enjoy and never has? Is it unpleasant because it is a bad story, poorly told, formulaic? Drop it.
Is it a book others have enjoyed over a wide span of time? Is it possible the attention span is lacking and not the book? Is it hard work? Keep it. Enjoyment is fleeting, and is not really the best standard for deciding whether or not to stick to a school book. Hard books that require a child to dig and labour are good material for growth.
Here is Charlotte Mason on the topic:
“I think we owe it to children to let them dig their knowledge, of whatever subject, for themselves out of the fit book; and this is for two reasons: What a child digs for is his own possession; what is poured into his ear, like the idle song of a pleasant singer, floats out as lightly as it came in, and is rarely assimilated.” (volume 3)
“Children must Labour.––This, of getting ideas out of them, is by no means all we must do with books. ‘In all labour there is profit,’ at any rate in some labour; and the labour of thought is what his book must induce in the child. He must generalise, classify, infer, judge, visualise, discriminate, labour in one way or another, with that capable mind of his, until the substance of his book is assimilated or rejected, according as he shall determine; for the determination rests with him and not with his teacher. (volume 3)
Perhaps it is not wholesome or quite honest for a teacher to pose as a source of all knowledge and to give ‘lovely’ lessons. Such lessons are titillating for the moment, but they give children the minimum of mental labour, and the result is much the same as that left on older persons by the reading of a magazine. We find, on the other hand, that in working through a considerable book, which may take two or three years to master, the interest of boys and girls is well sustained to the end; they develop an intelligent curiosity as to causes and consequences, and are in fact educating themselves. (also volume 3)
But boys get knowledge only as they dig for it. Labour prepares the way for assimilation, that mental process which converts information into knowledge; and the effort of taking in the sequence of thought of his author is worth to the boy a great deal of oral teaching. (volume 3)
It’s easy to be attentive when we love the book and cannot wait for the next part. It requires some more maturity and strength of character to stick to a book that is worthwhile, but we simply do not enjoy it.
“Hard ideas define a culture — that of serious reading, an institution vital to democracy itself. In a recent article, Stephen L. Carter, Yale law professor and novelist, underscores “the importance of reading books that are difficult. Long books. Hard books. Books with which we have to struggle. The hard work of serious reading mirrors the hard work of serious governing — and, in a democracy, governing is a responsibility all citizens share.”” (http://chronicle.com/free/v55/i39/39b01001.htm)
It might also help your student to know if a book was never written for children. Robinson Crusoe is like this. DeFoe wrote the story for adults, and he was as surprised as anybody when children took to it and called it one of their own. A number of books we think of as children’s books got their start that way, once upon a time. The book was published 300 years ago, and it is something marvelous to be in touch with one of the great literary minds of 3 centuries ago. It’s going to be a bit of work, but that work is well worth the effort, and your young scholars will find themselves stretched by it, but if one quits it because she doesn’t enjoy it, she will not get that stretching, and next year will be all the harder for it.
Something else that I think makes these works intended for adults by adopted by the children especially valuable to read is their outlook. Because they were written for adults, they have a grown up point of view, a mature way of looking at life and people. They stretch a child in a way that today’s books written for children just don’t, although certainly today’s children’s books are far more amusing and entertaining than yesteryear’s. There’s nothing wrong with reading books for fun, for entertainment. If I am hungry and it’s the middle of the day and I have an otherwise good diet, it’s not particularly harmful if I have a couple of cookies instead of a plate of raw broccoli. But if I eat only sweets because I have never learned to appreciate any vegetables or simple but nourishing foods such as pumpkin soup, roasted vegetables, cucumbers and cherry tomatoes, or a nice bit of roasted fish, then that is a problem.
What children in the past enjoyed, they can be brought to enjoy again (and so can the adults)- stretch them a bit, give them something to grow on.
I will also share my experience with making a child continue with a book they are not enjoying- one of mine, when introduced to Plutarch, began the year by crying whenever I got out Plutarch. We changed how we approached it, but we did continue. I shortened readings, did some more careful background introductions before each reading (this was before Anne White’s lovely study guides), had her read with a bookmark with key names and definitions written down, and so forth. By the end of the year Plutarch was not only her favourite book, she didn’t even remember that she had hated it at the beginning of the year.
I am not saying that it is always a mistake to drop a book from your schoolyear. Sometimes it is the right thing to do. I am saying the children’s current tastes are not the best standard for school readings. We are seeking to educate, broaden,and inform those tastes.