Check out the links below for the free download of the introductory books on the six systems of Indian Philosophy
Nyaya and Vaishesika – Tarka Sangraha by Annambhatta – https://fdocuments.in/reader/full/tarka-samgraha-with-english-translation
Sankhya – Sankhya Karika by Isvara Krsna with Commentary by Gaudapada – https://kupdf.net/download/the-samkhya-karika-english-translation_5b05e01ee2b6f514309b0b3d_pdf
Yoga – Patanjali Yoga Sutras with the commentary of Vyasa – https://www.theosophytrust.org/Online_Books/Yoga-Sutras-of-Patanjali_eBook-V1.4.pdf
Purva Mimamsa – Artha Sangraha – https://ia601901.us.archive.org/19/items/artha-sangraha/Artha%20Sangraha.pdf
Uttara Mimamsa or Vedanta – Vedantasara by Sadananda – https://estudantedavedanta.net/Vedantasara-Nikhilananda.pdf
(see also Lectures on Vedantasara by Swami Sarvapriyananda – https://www.youtube.com/playlist?list=PLDqahtm2vA70ccqIRFR_lipqKvxrHBRRw)
Or, see their downloads here:
https://drive.google.com/drive/folders/1ulWVxiC2Q847mpF4v0zAL_SgIzXAbQXD?usp=sharing
Know Thyself 