From "Let's Learn Hiragana" by Yasuko Kosaka Mitamura, p32: "The pronunciation of the Japanese [r] deserves careful attention because it produces a sound not found in English. It might be considered a cross between [r] and [l]. This sound is articulated by saying [r] while lightly touching the ridge behind the upper teeth once with the tip of the tongue, producing a flap [r]. This constrasts with curling the tip of the tongue back to produce the retroflex English [r]."
Er. WTF? I already knew that the Japanese (and Korean) "r" is somewhere between English "r" and "l", but I can't make any sense out of the last two sentences in that paragraph at all. I don't think it would have killed them to have simply included some photographs or diagrams here :/
The book is also going on about unvoiced, voiced and semivoiced consonants. Now, I get that the unvoiced consonants are the regular hiragana, the voiced ones have a " after which changes the meaning (e.g. hi --> bi), and the semivoiced have a circle after (hi --> pi) - but I have no idea why they're called "unvoiced", "voiced" or "semivoiced"! It's just assumed that you understand those terms - which would be great, if this wasn't a teach yourself Japanese-type book. And I have a feeling even were I to look up all the terms in Wikipedia or wherever, I still wouldn't understand, as I understand all the words in "saying [r] while lightly touching the ridge behind the upper teeth once with the tip of the tongue", but still have no actual clue what it means.