Good translations are few and far between of Coptic materials. HH Pope Shenouda's many booklets and books are usually available in English, but often very poorly translated, sometimes to the point of being confusing (and I say this as an OO person whose first language is English).
Exceptions to this would be the few books available from major EO publishers (SVS, etc.) of the works and sayings of Fr. Matthew the Poor (Matta El Meskeen), such as The Orthodox Prayer Life and others. Fr. Matthew is hugely influential on modern Coptic spirituality (he was the father who the future Pope Shenouda II confessed to while a monk back in the 1950s, and he nearly became Pope himself at the time of the election of HH St. Pope Kyrillos VI), so I would definitely recommend him (though it seems like many EO already know him and respect him, so maybe that's an obvious recommendation by this point).
From the last few hundred years...ehh...I think we are actually in better shape for English translations of more medieval writers than anything newer than that, unless the other churches in the communion buck this trend and I just don't know about it, which is entirely possible. Like I have some English translations of Severus Al Ashmunein (10th c.), Ibn Kabar (14th) and others, but there's a bit of a blind spot after that. Most of what I've seen from the 15th-19th centuries is by Westerners writing about the Church, or saints' biographies. I think you'd probably have to know Arabic to read what the Coptic Orthodox Church was producing at that time, though some of the letters of the Popes of that period have been preserved at various places, like HH Pope Demetrios II's (d. 1870) Papal bull against Protestantism (available via the Return to Orthodoxy website which was set up to combat the Protestant influences that are hounding the OO in our time as we struggle to figure out how to adapt to being in the West in large numbers for the first time in our history without being completely consumed by nonsense). Not sure how much things like that qualify as spiritual works, however.
Sorry I can't be more helpful. I think it is sad that a lot of what is preserved of OO works in English are the polemical works of saints like Mor Bar Salibi, Ps. Zecharias of Mytilene, and others. These too have their place, but they are not the only things that these people wrote! And of course if you are willing to spend a lot of money, you can find more in academic circles, but even those tend to treat the OO church as a bit of a black hole from about the 15th century to the modern era (not coincidentally very well timed with the European 'discovery' of the Copts et al. via the Roman Catholic reunion council of Florence; bleh).