In the Byzantine law book Corpus Iuris Civilis, the following sentence appears:
Iuris præcepta sunt hæc: honeste vivere, neminem laedere, suum cuique
tribuere. The precepts of law are these: to live honestly, to hurt no
one, and to give to each their own.
Thus the phrase "per praecepta iuris" comes to mind. (Praceptum is a second-declension neuter noun). The phrase "per praecepta iuris" means according to the rules of law.
You could also say, "per praecepta", which means, according to the precepts, or according to the rules. It often has the meaning, "according to the rules (precepts) of personal conduct."
In the same law book we get the passage:
Iustitia est constans et perpetua voluntas ius suum cuique tribuens. Iurisprudentia est divinarum atque humanarum rerum notitia, iusti atque iniusti scientia. Justice is the constant and perpetual desire to give to each their own. Jurisprudence is knowledge of things human and divine, knowledge of what is just and what is not just.
There are so many good passages in this law book (I'm using Wikiquote as my source) that I'll reproduce another.
Iure enim naturali ab initio omnes homines liberi nascebantur. For by natural law, all people were born free from the beginning.
We see that in Roman and Byzantine law, the words ius (law), iure (by law), de iure (according to the law), praecepta iuris (precepts of law), and auctoritate iuris (by the authority of law) often appear.
The idiom "by the book" can mean "by the rules" or "in the proper way", which is one reason I am searching through Roman and Byzantine law books for suitable phrases.
We can also use the phrase "secundum praecepta", which is found in Vatican literature. It means "according to the precepts" or "according to the teachings".
It is important to point out that the word "praecepta" comes from the Latin verb "praecipio", which means to teach. Thus praecepta means teachings.
To do things by the book means to do things by the rules, the way that they were taught.
- Secundum praecepta iuris. (According to the rules of law.)
- Per praecepta iuris. (According to the rules of law.)
- De praeceptis iuris. (By the rules of law.)
A few more suggestions:
- Secundum praecepta libri. (According to the rules of the book.)
- Per praecepta libri. (According to the rules of the book.)
- De praeceptis libri. (By the rules of the book.)
I would also like to point out that the phrase "per librum" might be a fine translation of "by the book". It is a natural choice for translating the English idiom, and even if it was never used in classical times, it is important to add new idioms to the Latin language.