Why does Bill hate farming books in The Day of the Triffids?

From the text:

At the same time I was endeavoring to learn the A B C's of farming. It is not the kind of thing that is easily learned from books. For one thing, it has never occurred to any writer on the subject that any potential farmer could be starting from absolute zero. I found, therefore, that all works started, as it were, in the middle, taking for granted both a basis and a vocabulary that I did not have. My specialized biological knowledge was all but useless to me in the face of practical problems. Much of the theory called for materials and substances whieh were either unavailable to me or unrecognizable by me if I could find them. I began to see quite soon that by the time I had dismissed the things that would shortly be unprocurable, such as chemical fertilizers, imported feeding stuffs, and all but the simpler kinds of machinery, there was going to be much expenditure of sweat for problematical returns.