For most of my recent scans, I have saved the original raw files, which are either 300, 400 or 600 ppi, depending on the book and when I scanned it. Note that these are raw, unedited scans, before any straightening, cropping, retouching, color correction or other edits have been performed.
If anyone wants copies of these files, they are available for the cost of a blank DVD and postage (the files are quite large, because they are saved as uncompressed TIFF or PNG files. Therefore an entire book takes up the better part of a DVD-R.) I don't currently have a list of books whose raw scans I have archived, but feel free to message me if you are looking for a particular issue and I'll see if it's one that I have.