The preamble does not exclude erasure of circled cells. If they are within the block of used cells and not needed they should be erased.
I liked this puzzle. I think the jigsaw effect is interesting and I have not seen it before.
I had an interesting side journey into American garden design before the correct theme emerged. You can get an initial and surname of a famous female landscape gardener from the circled letters.
I made the end game harder by not initially appreciating the mirrored segment is not central in the grid. I spent some time looking at symmetrical pairs of letters and excluding those where one or both did not appear in the quotation. The method is fine but you must do it around the centre of the used cells not the centre of the grid.