I agree with chrisspillertkd - if you are looking specifically for ITF information, the items he lists are considered the standard reference sources for the Ch'ang H'on (ITF) tul set and their applications. I would invest in one or more of those before buying something else that's not on his list - not because other references are poorly done (although some are - not the ones on this page, but others) but because those are the ones that most, if not all, ITF practitioners use as reference points, while many of the others, regardless of quality, do not necessarily emphasize techniques the same way - not good, not bad, not right or wrong - just differently, and that could lead to confusion.