At the end of The Problems of Philosophy, Bertrand Russell says:

The student who wishes to acquire an elementary knowledge of
philosophy will find it both easier and more profitable to read some
of the works of the great philosophers than to attempt to derive an
all-round view from handbooks. The following are specially
recommended:

• Plato: Republic (especially Books VI and VII)
• René Descartes: Meditations on First Philosophy
• Baruch Spinoza: Ethics
• Gottfried Wilhelm Leibniz: Monadology
• George Berkeley: Three Dialogues Between Hylas and Philonous
• David Hume: An Enquiry Concerning Human Understanding
• Immanuel Kant: Prolegomena to Any Future Metaphysics