Well, I first have to admit that I studied German for only one year at school and have forgotten nearly all of it. :sorry:
But, because I can never resist poking my nose in...
I found German grammar quite difficult because it has four different cases, which English does not have. But the pronunciation isn't complicated (I mean, to read from what is written). I think it's harder than Japanese, but it isn't excruciatingly difficult.
Verbs are at the end of the sentence, like Japanese.
It has tenses, like English and French, and nouns change as well as verbs.
About the books and the use of German, I have no idea.
That is just my two cents worth...
And as to how easy it is to learn, it really depends on you... I know some people who have started and learned German quickly and found it quite easy, whereas other people find it hard.
When my Japanese has become more advanced, and I'm no longer paying so much time to it, I am considering learning German.
(At present I speak only French and English
)