There are plenty of free resources on the WWW for learning all kinds of languages. The difference is all in the formatting. My observation is that the actual hard-copy texts that I have seen present long, complicated lessons that throw tons of vocabulary at the learner. I found the Teach Yourself series a good fit for my learning style, but these books require the learner to absorb a great deal of vocabulary and other information from each chapter. (Teach Yourself Gaelic proved a disappointment, especially without the accompanying CDs, because the book never explores the correct pronunciations for the many diphthongs and triphthongs for which Gaelic is infamous.)
My approach is to scale it back and provide concise two-page lessons, each with about a dozen new words and at most one or two new grammatical concepts. Only the very first lesson, devoted mostly to pronunciation, is a double-length episode. Each unit is divided into a dozen lessons, including a vocabulary review, plus some bonus material. Busy learners can master small amounts in small chunks of time, more easily, at their own pace, before moving on to the next lessons.