Badarayana. The Brahma Sutra: The Philosophy of Spiritual Life. Tr. and with an introduction and notes by S. Radhakrishnan. New York: Greenwood Press, 1968.
Coulson, Michael. Sanskrit: An Introduction to the Classical Language. Chicago: NTC, 1992. Been teaching myself Sanskrit with this.
Coward, Harold G. and K. Kunjunni Raja. The Philosophy of the Grammarians. Princeton: Princeton University Press, 1990. gitfiddle, since you’re interested in both Panini and philosophy, you must see this book.
Grimes, John. A Concise Dictionary of Indian Philosophy: Sanskrit Terms Defined in English. Albany: SUNY Press, 1989. You’ll really like this one, gitfiddle.
Guénon, René. Man and His Becoming According to the Vedanta. A good introduction, but caution with Guénon, because the intellectual neo-Nazis are glomming onto him. Guénon wasn’t a fascist but fascists like him for some reason.
Monier-Williams, Monier. A Sanskrit-English Dictionary. Reprinted: Delhi: Motilal Banarsidass, 1995.
Radhakrishnan, S. Indian Philosophy. Delhi: Oxford University Press, 1923 (reprinted 1997). IMHO still the best introduction of all to the subject.
Schweitzer, Albert. Indian Thought and Its Development. London: A&C Black, 1951.
A Sourcebook in Indian Philosophy. Ed. by Sarvepalli Radhakrishnan and Charles A. Moore. Princeton: Princeton University Press, 1957.
Zimmer, Heinrich. Philosophies of India. New York: Meridian, 1951.
Panini’s Ashtadhyayi is about Sanskrit grammar. The first real grammar ever written. If you’re really into Sanskrit and philosophy