The appearance of koan collections in the Song dynasty marks a shift in Zen teaching methodology or skillful means. It is a move towards refined literary activity, which was termed "lettered Ch'an" or moji Zen. Moji Zen was not a real advance in terms of skillful means. It eventually led to a correction because such Zen was fast becoming more about poetry and making clever allusions to satori than personal accomplishment of actual enlightenment. On the same score, one might say the same thing about trying to accomplish enlightenment by means of sitting meditation—just sitting on a cushion. But how close or how far away are such practices when compared with actually seeing pure Mind? I think one could argue that the means was turning into the end—the raft to the other shore was becoming the other shore!
Koans and sitting meditation might be useful tools in our Zen tool box but they are not the only ones. Reading the Buddha's discourses might be just as valuable if not more since these discourses are composed in such a way as to alter our typical Western way of reading and thinking about texts forcing us to drop our former patterns of thought. According to the Lankavatara Sutra,
Mahamati, owing to the functioning of the minds of sentient beings [. . .] religious discourses are given by myself and other Tathagatas, Arhats, Fully-Enlightened Ones in response to varieties of faiths on the part of beings, in order to remove them from [the bondage of] the Citta, Manas and Manovijnana, and not for the attainment and establishment of self-realisation which issues from noble wisdom.
In other words, the discourses of the Buddha are not given to establish self-realization but to remove beings from bondage to thoughts, fancies and discursive thinking. A good example of removing beings from bondage to thoughts, etc., can be found in the Śūraṅgama Sutra which, incidentally, has been influential in the study and practice of Zen. This particular discourse helps us to distinguish between the false, conditioned mind of ordinary beings—what I like to call the monkey-mind—and the highest, or unconditioned Mind. We often forget and take it for granted that the mind we have and use to understand the highest Mind, is the false mind. Śūraṅgama Sutra proves to be frustrating for not only the beginner but also the seasoned veteran who still has not managed to awaken to the unconditioned Mind. The natural tendency is to keep using the conditioned, false mind to prepare the way for the intuition of the true Mind which never happens. But this is not all that bad if it leads to going to one's wits' end which I have mentioned before.