These books helped me: Garden of Emunah by Rabbi Shalom Arush, Chizuk by Rabbi Eliezer Parkoff, Trust Me by Rabbi Eliezer Parkoff, Thoughts for a Jewish Heart by Rav Ezriel Tauber.
I would also recommend Derech Hashem by the Ramchal. In order to really strengthen our emunah, we have to have, or at least try to have, an understanding of Hashem and His goodness, and His relationship to us. I think this is basic and should preceed any other reading. Hatzlacha!