First, please note my corrections to your post above, marked in red. Start every new sentence with a capital letter. Don't put a space after opening quotation marks. Your sentence starting "Kindly explain ..." was not a question so a question mark was not appropriate. There is no need to put your name at the end of a post. We can see your username. There is also no need to address posts with things like "Dear teachers". You can just write "Hi" or "Hello" if you feel you must use anything at all.
There is very little difference between your two sentences. There is a slight nuance of difference - "He is in the library" can only mean that he is inside the building. "He is at the library" could mean that he is standing outside it.
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