If there was a book that had all of the Secret Teachings of Theravāda, The Path of Purification (The Visuddhimagga) would be that book. It explains all the good stuff, like how to attain samādhi concentration; how to attain psychic powers including knowing and seeing the past lives. Lastly, it includes the stages of insight and wisdom that one needs to reach Nibbāna. Books do not get more complete than this. You can read a free and condensed version called Knowing and Seeing by The Most Venerable Pa-Auk Sayadawgyi who teaches this exact method based on The Path of Purification.
You can find The Path to Purification on free pdf link here . If you would like to know more about mindfulness of breathing. Bhikkhu Nyanamoli wrote a small book which contains information from both the Visuddhimagga and Vimuttimagga. It can be found here at this free web link: or in book format at this amazon link
Basically, there are no secrets in Theravāda. There are only texts that are not very well known (until now). You should never have to pay for such teachings and be warned if someone asks you to pay for such teachings. All of the content in the book Knowing and Seeing can be learned at Pa-Auk Meditation Centres. There is no charge at these centres, even for food and lodging. Best of luck and let me know how things go.
Below is a wikipedia excerpt of The Path of Purification.
The Visuddhimagga (Pali; English: The Path of Purification), is the ‘great treatise’ on Theravada Buddhist doctrine written by Buddhaghosa approximately in the 5th Century in Sri Lanka. It is a comprehensive manual condensing and systematizing the theoretical and practical teachings of Gautama Buddha as they were understood by the elders of the Mahavihara Monastery in Anuradhapura, Sri Lanka. It is described as “the hub of a complete and coherent method of exegesis of the Tipitaka, using the ‘Abhidhamma method’ as it is called. And it sets out detailed practical instructions for developing purification of mind. It is considered the most important Theravada text outside of the Tipitaka canon of scriptures.
The Visuddhimagga’s structure is based on the Ratha-vinita Sutta (“Relay Chariots Discourse,” MN 24), which describes the progression from the purity of discipline to the final destination of nibbana in seven steps. The Visuddhimagga’s material also strongly resembles the material found in an earlier treatise called the Vimuttimagga (c. 1st or 2nd century).