Dear Prof. Pandurangi,
Sorry that this is not the book you were looking for. Hopefully someone here will recall it.
In the meantime, I can mention a few older books related to this topic.
The Buddha-Mimansa: or, The Buddha and His Relation to the Religion of the Vedas, by Yogiraja's Disciple Maitreya, 2nd ed., London: Thacker, 1925; several reprints.
The Doctrine of the Upaniṣads and The Early Buddhism, Die Lehre der Upanishaden und die Anfänge des Buddhismus, by Hermann Oldenberg, translated into English by Shridhar B. Shrotri, Delhi: Motilal Banarsidass, 1991 (German original, 1908).
Upanisads and Early Buddhism, by Sanjay Govind Deodikar, Delhi: Eastern Book Linkers, 1992.
Best regards,
David Reigle
Colorado, U.S.A.