Student or Learner
This is a thank-you letter for the coworkers of a book, and I can't understand what "plagiarism is the sincerest form of flattery" means. Is the writer trying to say some coworker plagiarized something that contributed to the work?
ex)Neil MacGregor and Patricia Williams are the true fathers of this Guide, and its first readers. I cannot thank them enough for their encouragement and their perceptive and challenging comments....I hope they will bear in mind the old saying that plagiarism is the sincerest form of flattery...It has also been a privilege to work with Diana Davies, the most scrupulous and caring...
I can only guess that the author took many of Neil's and Patricia's suggestions word-for-word. Given the context, he's clearly not talking about the illegal type of plagiarism.
The expression I know is "imitation is the sincerest form of flattery." If you act like me, it means you admire me.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.