"I read an interesting book."
面 is primarily FACE, though it can indeed also mean "mask", as well as "surface/plane" or "side", depending on context and attached kanji.
Not sure if this is etymologically sound, but I suspect "white face" came to mean "entertaining" thanks to actors/performers and geisha.
The following answer is from Japanese Stack Overflow by Teno
According to 語源由来辞典 ( http://gogen-allguide.com/o/omoshiroi.html ), 「面白い」 is originated from 「面白し」. 「面」 used to mean "a sight/view" (the source says the front of eyes) and 「白い」 used to mean "bright and clear." Then 「面白し」 later came to mean "a light/bright sight/view" and then later "a beautiful sight/view". It further came to mean "fun" or "comfortable", which represents a pleasant feeling.
You're misunderstanding the grammar pattern. おもしろい is the base adjective. The only times you would conjugate it for past tense are, yes, when making the descriptor itself past tense (implying that it used to be interesting), or when preceding です.
この本はおもしろかったです。 This book was interesting. It still is, but the tense suggests rather that it was an interesting read.
このおもしろかった本。This once-interesting book. An adjectival which precedes a nominal (noun), no matter what tense that adjectival is in, always describes that nominal.
この本はおもしろいでした。 NEVER do this. A past-tense い-adjectival immediately preceding the predicate (です) is always conjugated as ~かった(です).