The correct sentence is: "Is there anything in life so disenchanting as attainment?" mused author Robert Louis Stevenson. The word 'mused' is not capitalized as it follows a quotation and is part of the same sentence.
The corrected sentence with proper capitalization and punctuation would be: "Is there anything in life so disenchanting as attainment?" mused author Robert Louis Stevenson.
The rules of capitalization require that 'mused' should not be capitalized since it follows a quotation and is part of the same sentence.
Additionally, the punctuation mark at the end of the quotation should be a comma if it is a part of a larger sentence that continues beyond the quote, but here it is appropriately a question mark since the quotation itself is a question.