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The problem isn't grammatical but idiomatic. The correct idiom is "years of
effort," so saying "years of efforts" would immediately tag this piece of text
as non-English or, possibly, non-American English, to a native speaker.
If you want printed backup, I'd look quickly through the Longman section of your
closest technical bookstore or check out the books about English as a second
language in the library. Longman (U.K.) publishes idiom-aware dictionaries for
ESL teachers and students.
sfowler -at- ejv -dot- com