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My girlfriend is from Fujian and speaks 闽南语, so I am trying to learn so I can communicate with her family better.
The struggle is finding any useful resources (even in Chinese), so I'm wondering if anyone has any good suggestions for sites, books, or anything else to improve my skills?
There is an American guy (I think) who teaches people how to speak Taiwanese on Youtube. He even teaches the native Taiwanese people how to speak 闽南语. You can see the videos here.
EDIT: Looks like the playlist is dead. This is the guy https://www.youtube.com/watch?v=lJo2OHi6hgs
Go to the most reliable source -- order the Maryknoll Fathers' set of 3 textbooks with CDs. While it is not exactly comprehensible input in its best form (it's predictible, since it's a book and the order doesn't change), it is the best set of materials currently available, and certainly the most comprehensive. If you have the patience to work through all three volumes, you'll have a very good basic knowledge. (Maryknoll has a really far-reaching knowledge of Taiwanese language...As they told me at their Taipei language center once: "Well, let's get going. When we finish these three books [each one is like an inch thick], we can really start learning the language!")
The Minnan-English and English-Minnan dictionaries of theirs, which is excellent, is available for free download as well: http://www.taiwanesedictionary.org/
XiaMen University publishes a couple books on MinNanHua, they use their own pinyin which will make sense if you have studied some Mandarin but in any case is often clearer than the Peh-oh-je the Taiwanese missions use. The amazon.cn link is here (Not an associate link)
You can also look at these:
There is a more recent textbook (published after the other answers were written!), called "Southern Hokkien: An Introduction", by Bernhard Fuehrer and Yang Hsiu-fang. It focuses on Taiwanese Hokkien. It's in English, makes many comparisons with Mandarin, and is well written.