Several fonts (especially ones from Adobe) are offered in a range of variants to suit different sizes, typically 'Caption' (for small sizes), 'Regular' (for 11-18pt) and 'Display' (for larger sizes). The Caption variant is often well-suited to use in an ereader, offering a slightly bolder font, reduced stroke contrast and slightly more open apertures (aperture refers to things like the gap between the crossbar and lower stroke on the right of an 'e' and is important for letter recognition). I think these are generally a better choice than the semibold weight, which is often too heavy.
Caecilia may be a very readable font, but I would find its abysmal ugliness far too distracting. You need to strike a balance between the elegance of the letterforms and the screen's ability to display them properly.
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