"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.
I'm pointing out that "read" can be read as both present and past tense in English, making how to translate it into Japanese ambiguous.
Present tense in English is a little awkward, but the japanese translation of it isn't.
Think of it as "I read a book that is interesting."
If you wanted to say "I read a book that used to be interesting", then you could use おもしろかった本を読みました.
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).
Note the ambiguity in the English sentence.
The word "read" can be pronunciated as "reed" or "red", which signify the present tense and past tense respectively.
Can 面白い and楽しい be used interchangeably. Cause i kearned that Tanoshii was fun and omkshiroi is interesting.
読み [yomi]: (P, n) reading (noun)
読む [yomu]: (P, v5m, vt) to read, to count (now mostly used in idioms), to guess, to predict, to read (someone's thoughts), to see (e.g. into someone's heart), to divine
Isn't 面白い used to describe something that is funny rather than interesting? I used it once to describe someone's drawing and they got a bit ofdended...
I think of おもしろい as meaning "out of the ordinary"; it could be out of the ordinary interesting, out of the ordinary funny, out of the ordinary weird... Maybe that person thought you were calling their drawing weird :P