>>858
I feel "mitsukaru" is like "to be found" in English and "to find" is "mitsukeru."
So, if the question is "nihongo no hon __ mitsukeraremasen," then the answer is が. These examples may help you understand:
I found the book. 本を見つけた。
The book was found. 本が見つかった。
>この漢字の読み方（ ）わかりますか
>I'm pretty sure the answer is が, I just don't know why. I'm hopeless with grammar...
I don't think it is very important since you already know "わかる" always follows "object+が/は" (or you can drop が/は in spoken language). Linguists may explain why you can't use を in this case. But it's like grammar jargon can explain why you say "I know how to read this kanji" but don't say "I'm knowing how..." I was frustrated with this kind of English grammar and felt hopeless just like you do, but, after a few months of learning, somehow "knowing" started to sound strange though I don't know the underlying grammar.