Once More With Footnotes was a special release for a US Convention (WorldCon?) where Terry was Guest of Honor (note: no u)
The first print run was very limited, and sold out quickly. They only did one more print run, hence its limited availability.
It has a collection of fiction /and/ articles written by Terry, so there is stuff in OMWF which isn't in Blink of the Screen, but it's only the non-fiction stuff. However, there's a collection of the non-fiction stuff coming out at the end of the year (Tap of the Keyboard? Can't remember the title) which will probably have all the articles from OMWF plus many more.
So... you can probably live without it