Zwingli’s most accessible (for the person just wishing to familiarize him or herself with his theology) is, in my estimation, his Eine kurze christliche Einleitung, which he published on the 17th of November, 1523. As Schaff relates
[In 1523] The Council was not prepared to order the immediate abolition of the mass and the images. It punished Hottinger and other “idol-stormers” by banishment, and appointed a commission of ministers and laymen, including Zwingli, Schmidt and Judae, who should enlighten the people on the subject by preaching and writing. . Zwingli prepared his “Short and Christian Introduction,” which was sent by the Council of Two Hundred to all the ministers of the canton, the bishops of Constance, Basle, and Coire, the University of Basle, and to the twelve other cantons (Nov. 17, 1523). It may be compared to the instruction of Melanchthon for the visitation of the churches of Saxony (1528).
It proved to be quite influential. Fortunately for readers of English, the little volume has been included in this edition–
In fact, Wipf and Stock are the only publishers who have available any of Zwingli’s works in English. Go there, search for ‘Zwingli’, and you’ll find them.