J.R.R Tolkien is not just a writer and the Legendarium is not just a creation. Like Dante's Devine comedy or Homer's Odyssey - the Legendarium is a whole world that contains a lot of themes and ideas.
Anyone that read Shippey or Pearce can recite by heart the known themes that exist in LOTR: mythology (Norse, German or even Celtic), Christian, Linguistic or Philosophical theme (Aristo or Neo Renaissance influence etc.). But after you had learned those themes, you are spending a lot of time and effort to find some new themes or new ideas. There are a lot of books about Tolkien and most of them discussing the known themes.
If you want book about those themes, A Question of Time is not the book for you. If you are a novice Tolkien researcher, you should read Shippey, Pearce or Anderson and don't start with this book.
This book contains different themes and ideas. The main theme in this book is about time and dreams, an idea that I never thought about or read it elsewhere. Flieger shows us that LOTR and other Tolkien's creations have a grain of time and time travel inside of them. She backs her theories with powerful examples from LOTR drafts (History of Middle Earth vol. 6-9) and from Tolkien's time travel story (Vol. 5 and 9).
Flieger does convince me that Tolkien thought about time and incorporated his thoughts in LOTR. I was amazed that after reading and researching Tolkien for such a long time, she actually told me something new, something that made me read LOTR and the Legendarium in a different perspective. I had the same feeling after I read Shippey's book and I am sorry to say that few of Tolkien criticism books are giving me the same sense.
To summarize my words: Read Shippey, Pierce, and Hammond & Scull and of course Anderson's annotated Hobbit first. But If you have read those already and you are searching for a new theme - READ THIS BOOK!