Let me second the recommendation for The Road to Middle Earth -- it's the single best work of literary criticism I've ever seen.
Also, given the depth of interest about Tolkien and his craft that you have already expressed, TRtME is a much better choice than Shippey's later J.R.R. Tolkien: Author of the Century, which has a different focus and treats it at a more popular level. TRtME is essential for understanding what exactly Tolkien saw himself as trying to achieve, and how he in fact achieved it. (IMHO)
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Also, given the depth of interest about Tolkien and his craft that you have already expressed, TRtME is a much better choice than Shippey's later J.R.R. Tolkien: Author of the Century, which has a different focus and treats it at a more popular level. TRtME is essential for understanding what exactly Tolkien saw himself as trying to achieve, and how he in fact achieved it. (IMHO)