It is very easy to name a book or give a link. So. let me give a little understanding of the historical events and based on this please make your own choice.
Arjuna was well versed in the Scriptures of that time and all the major philosophies as he had many teachers, relatives, and friends. It’s to him Gita was bestowed by Krishna. Gita and Upanishads and Tantra are secret literature called Rahasya that is passed on from Guru to a student. Many of us don’t fall into the caliber of Arjuna nor do have Gurus who can give us initiation in-person and years’ worth of training. So does that mean we are no eligible to read it? Of course, we are eligible, but let’s explore a little more.
During Sri Krishna’s time, only a few people knew Sri Krishna was an avatar of Narayana and a Rudra himself. People at that time followed Vedic Schools (like Shakila, Madhyandina, and Kanva), and each school had its own Upanishad. They also had various philosophies like Sankhya.
When the young boy (Adi Shankara) came, Buddhism was very popular and widespread. He also noticed the Sanatana culture had too many divisions and too many Vedic schools. Even Gautama before becoming Buddha went to many schools but was not pleased. So this young boy selectively took the Upanishad of many Vedic Schools and linked them with Sutras, Tantra, and Gita. Why? Because he didn’t want people to climb different branches and throw trash at each other. 1000 years after Adi Shankara many sectarian acharyas took the same literature and gave their own version of translation and commentary. For example, If they love Krishna, they wrote everything with Sri Krishna as the center, the same goes with other sects. They are like fan clubs, but nothing wrong with doing so as the intent was to foster bhakti in a focused direction.
The conclusion is, one can’t read this literate, one has to explore it since the goal is not to finish it. This is not a course that one has to complete in a year and pass the exam. It’s like eating food, one can’t rush food into the mouth, same with sleeping, one can’t rush sleeping. So, please try this link, but do your own exploration that will connect the wisdom of various literature then come to your own conclusion and practice. Find books that have detailed explanations and references to the Vedas and Upanishads. Get the vocabulary of the core Sanatana culture. Good day.
Here is an example, click the 2nd chapter called Sankhya Yoga and at the end, you will seek a detailed reference of each hymn with Vedas : https://www.shankaracharya.org/gita_bhashya.php
Here is a link to IIT Kanpur Unversity that collected various translations into one location https://www.gitasupersite.iitk.ac.in/
Vocabulary of Sanatana Culture: Sanatanadhara.com
Some understanding of Vedaic darshanas : http://namahshivaya.net/darshana/
Image reference: https://www.bhagavad-gita.us/the-bhagavad-gita-in-audio-english/