Texts like Moreland & Fleischer, Learn to Read Latin, and Self-Education in Latin make use of unadapted readings as one gets past the early chapters. However, I’ve made it a point to seek out texts that introduce unadapted ancient readings early (preferably lesson 1). Those that I’ve found all make use of Caeser:
An Inductive Latin Method by Harper & Burgess, 1883
An Inductive Latin Primer by Harper & Burgess, 1891
Both of these start with an interlinear and get you reading Caesar from lesson 1. The Primer is probably the easier of the 2. I haven’t found answer keys to the exercises for either. The same authors later wrote the Elements of Latin, but dumbed it down with adapted readings in the early chapters.
Bellum Helveticum by Cornelius Marshal Lowe & Nathaniel Butler, 1894. Those who like the Lingua Latina approach of asking questions in Latin to be answered in Latin will like this. Even the pensa instructions are often given in Latin. From the preface, “The chasm between the beginner’s book and Caesar, which often renders advisable the use of some easier intermediate text as a bridge, does not exist when the student has been reading Caesar from his first lesson in Latin.”
Bellum Helveticum for Beginners in Latin by Cornelius Marshal Lowe & Nathaniel Butler, revised by Arthur Tappan Walker, 1900. This removes a few nice features, like calling the exercises “pensa” and giving most instructions in Latin. However, it still retains the questions in Latin to be answered in Latin. I prefer this one because it has an answer key. Later editions by Janes & Jenks started dumbing it down with adapted readings in the early chapters.
I Came, I Saw, I Translated: An Accelerated Method for Learning Classical Latin in the 21st Century by Drew Arlen Mannetter, 2011. I think this isn’t necessarily a bad book. The pedagogy seems sound. However, I don’t like that it is arranged to be its own workbook with blanks for students to fill in. It’s a recipe for college students to ruin the resale value of their textbooks and makes it harder to save money by making clean used copies scarce.
If anyone knows of any others, I’d love to hear about them.