Hi - this forum is mainly for support of SIL fonts, not the SIL Open Font License, however I’ll try to help.
Font embedding is discussed in the OFL-FAQ. I’ve copied relevant portions below. Unlike PDF, ebook packages mostly ‘embed’ fonts by copying the font into the package and referencing it in the ebook CSS. I can’t tell exactly what Calibre does, but it seems it may be doing it this way. This is the situation described in 1.15 below.
The key question is whether the fonts are changed at all by Calibre. If they are just copied in, then the license info remains in the font itself and you’re OK. If they change the fonts then you’ll need to confirm that the copyright and license info in the fonts remain there. You’ll need to write the Calibre developers to confirm what they are doing.
In any case, it’s a great idea to include a simple, short reference to the fonts and license in your epub, as in:
The font embedded in this epub is fontname, available at URL and licensed under the SIL Open Font License (http://scripts.sil.org/OFL).
Doing that should be enough, as long as Calibre is not stripping out copyright and license info. If they are, well, then they shouldn’t.
Hope that helps,
Victor
OFL-FAQ portions:
1.10 Does the full OFL license text always need to accompany the font?
The only situation in which an OFL font can be distributed without the text of the OFL (either in a separate file or in font metadata), is when a font is embedded in a document or bundled within a program. In the case of metadata included within a font, it is legally sufficient to include only a link to the text of the OFL on http://scripts.sil.org/OFL, but we strongly recommend against this. Most modern font formats include metadata fields that will accept the full OFL text, and full inclusion increases the likelihood that users will understand and properly apply the license.
1.11 What do you mean by ‘embedding’? How does that differ from other means of distribution?
By ‘embedding’ we mean inclusion of the font in a document or file in a way that makes extraction (and redistribution) difficult or clearly discouraged. In many cases the names of embedded fonts might also not be obvious to those reading the document, the font data format might be altered, and only a subset of the font - only the glyphs required for the text - might be included. Any other means of delivering a font to another person is considered ‘distribution’, and needs to be accompanied by any copyright notices and licensing information available in OFL.txt.
1.12 So can I embed OFL fonts in my document?
Yes, either in full or a subset. The restrictions regarding font modification and redistribution do not apply, as the font is not intended for use outside the document.
1.13 Does embedding alter the license of the document itself?
No. Referencing or embedding an OFL font in any document does not change the license of the document itself. The requirement for fonts to remain under the OFL does not apply to any document created using the fonts and their derivatives. Similarly, creating any kind of graphic using a font under OFL does not make the resulting artwork subject to the OFL.
1.14 If OFL fonts are extracted from a document in which they are embedded (such as a PDF file), what can be done with them? Is this a risk to author(s)?
The few utilities that can extract fonts embedded in a PDF will typically output limited amounts of outlines - not a complete font. To create a working font from this method is much more difficult and time consuming than finding the source of the original OFL font. So there is little chance that an OFL font would be extracted and redistributed inappropriately through this method. Even so, copyright laws address any misrepresentation of authorship. All Font Software released under the OFL and marked as such by the author(s) is intended to remain under this license regardless of the distribution method, and cannot be redistributed under any other license. We strongly discourage any font extraction - we recommend directly using the font sources instead - but if you extract font outlines from a document, please be considerate: respect the work of the author(s) and the licensing model.
1.15 What about distributing fonts with a document? Within a compressed folder structure? Is it distribution, bundling or embedding?
Certain document formats may allow the inclusion of an unmodified font within their file structure which may consist of a compressed folder containing the various resources forming the document (such as pictures and thumbnails). Including fonts within such a structure is understood as being different from embedding but rather similar to bundling (or mere aggregation) which the license explicitly allows. In this case the font is conveyed unchanged whereas embedding a font usually transforms it from the original format. The OFL does not allow anyone to extract the font from such a structure to then redistribute it under another license. The explicit permission to redistribute and embed does not cancel the requirement for the Font Software to remain under the license chosen by its author(s). Even if the font travels inside the document as one of its assets, it should not lose its authorship information and licensing.
1.16 What about ebooks shipping with open fonts?
The requirements differ depending on whether the fonts are linked, embedded or distributed (bundled or aggregated). Some ebook formats use web technologies to do font linking via @font-face, others are designed for font embedding, some use fonts distributed with the document or reading software, and a few rely solely on the fonts already present on the target system. The license requirements depend on the type of inclusion as discussed in 1.15.