Quality definitely varies by author, and outside the 60 Hours Heresy series there are some great other series like Eisenhorn and Gaunts Ghosts. And even humor like Caiphas Cain series.

On the use of old historical texts (esp Catholic) one my favorites is the Sabbatine Martyr connection from Gaunts Ghosts. Sabbatine in 40k is a Joan of Arc type character that dies a martyr (not a spoiler, ancient backstory in the books). A system is named after her, which sets up the title of one of the books “Sabbat Martyr” a play on the prayer/hymn “Stabat Mater”or “Sorrowing Mother”