The standard edition of the works of Spinoza in Latin is that of Dr Carl Gebhardt (Heidelberg, 1925); a more widely available edition is that of Van Vloten and Land in two volumes (The Hague, 1882).
English translations of the Ethics and of The Treatise on the Correction of the Understanding have appeared in Everyman's Library and in the Bohn Series; the two-volume edition of the Chief Works of Spinozain the Bohn Series also includes translations of the Theological-Political Treatise and of the Political Treatise. There is an earlier and better translation of the Ethics and On the Correction of the Understanding by W. Hale White (better known as 'Mark Rutherford') and Amelia S. Stirling.
The best English edition of the Letters, which are indispensable for an understanding of Spinoza, is The Correspondence of Spinoza edited by A. Wolf (Allen and Unwin, 1929), which has a useful introduction and notes.
The early Principles of Descartes' Philosophy, with its appendix, the Metaphysical Thoughts, is of importance chiefly to those studying the development of Spinoza's thought and writing. The early Short Treatise on God, Man, and his Well-being was translated and annotated by Professor A. Wolf; the Short Treatise is also primarily of interest to students of Spinoza's development.
The most careful studies of Spinoza in English are A Study of Spinoza's Ethics by H. H. Joachim (Oxford University Press, 1901) and the posthumously published lectures, Spinoza'sTractatus De Intellectus Emendatione: A Commentary . . .