Hi
Here are the results of some trials.
To begin, you'll find four (there are more of them)
otf fonts of the
Linux Libertine O family. Please note that these fonts are published under the GPL license, and that the further modified and renamed fonts below share of course the same license. They are in the folder: "A original fonts".
These fonts have been darkened using the nice script of Sherman Perry,
ReadifyFont.
I selected two different darkening levels from a range from 0 to 50 and modified accordingly their side-bearings: 30 is nearly a semi-bold value, while 20 is only neatly darker. I produced four ttf fonts, renamed
LibertineO-G30 or
LibertineO-G20. The Bold and BoldItalic fonts have
not been darkened. These fonts can of course be installed permanently on your system or side-loaded in any decent ebook-reader (like Koreader). They are in the folder: "B darkened."
These darkened fonts have been subsetted with
pyftsubset (a part of the
fonttools project) against the list of characters named
characters330 which contains the 110 unicode characters of the joint EPUB (the list is produced by the reports tool of the Calibre Editor). The subsets are in the folder: "C subsets."
I also joined the test EPUB with its subsetted -and darkened- fonts. It passes the epubcheck validation test.
About size
This way one can embed subsets of any font, including a customized one. Here the regular subset size used for the EPUB is about 44k, and the italic one, 37k (from a common 110 characters list).
As we make it work currently with a single list of characters, pyftsubset can provide a clear size saving advantage if the book use only both regular and italic fonts. This advantage could be greatly reduced, even nullified, if we had to include bold and bolditalics fonts.