This is mostly about koine, so it doesn’t apply to classical Studies.
I agree with Marcos, it was not perfectly obvious what “a straight forward Greek textbook” would look like. The order of presentation in old traditional (koine greek) pedagogy moved in the direction:
morphology → basic clause syntax → advanced syntax → exegesis (koine greek)
In the last 50 years there’s been a lot of innovation. William LaSor Fuller seminary
… William LaSor’s Handbook of New Testament Greek, it combines reading lessons (vol. 1) with grammar, paradigms, and basic vocabulary (vol. 2). William LaSor uses the inductive method, studying directly from the text, rather than the conventional method of language study in which beginning students learn the rules of grammar and syntax and memorize vocabulary, often without reading the actual text. Instead of memorizing numerous forms that will never be encountered in actual reading, the student learns only what he or she encounters
.
Eugene V N Goetchius, Language of the New Testament was one-of-a-kind. The author was a linguist who taught New Testament an Episcopal seminary in Massachusetts. He was a colleague of Edward Hobbs who taught at Wellesley. Decades ago Hobbs told me he used copies of Goetchius’ class notes while teaching Greek to Wellesley students. Goetchius’ methodology is heavily indebted to transformational grammar (pre-Chomsky).
More recently, there’s been a lot of Greek instructors moving into second language acquisition. Markos can tell you more about that. The traditional pedagogy hasn’t died. Mounce-Wallace revival of J G Machen is showing a little fraying around the edges, with a lot of competition being published in the last two decades.
There’s nothing straightforward about learning ancient languages. If you attended public school in North America (not the UK!) the probability of ancient language study was pretty low. It was an elective in high school 60 years ago, not too many people were electing it. The idea of learning Latin structure terror in the hearts our students who are struggling with modern French or Spanish. Ancient Greek wasn’t even an option.
The classical school movement founded by Doug Wilson in Moscow, Idaho reintroduced classical languages into the curriculum at the primary level.