I must be the last person on the planet to know this, but Fr Walter Farrell, OP’s, famous Companion to the Summa, is on-line for all to consult. You can use the thing as a handy on-line book, or simply download it for use on your own computer. It covers all the matter from the Summa theologiae in a conversational yet authoritative way.

Obiter dictum: Fr Farrell (1902-1951) is buried in All Saints Cemetery in Des Plaines, Illinois, next to the graves of Humbert Kane, OP, Raymond “Jude” Nogar, OP, and my beloved James A. Weisheipl, OP.