I never read any English textbooks on psychophysics, so I cannot help you there. If I wanted to find a good one, I would look at the curricula at some universities and see which books they use or recommend. Such information is often available online.
Beyond a textbook, like user1406647 (+1), I find it enlightening to read classic experiments by the inventors of psychophysics. Their texts are easy to understand without much previous knowledge, and the experiments are simple and foundational for the understanding of the principles of psychophysics. Also, I believe that textbooks only ever give a vague and superficial idea of a field, while confrontation with original texts provides a deeper insight. This is especially true for the presentation of theories: in textbooks they are always abbreviated beyond recognition. Textbooks are good in that they provide a possible structure (e.g. they group authors and theories), delineate historical development, and summarize critique. They are like maps: you'll have to do the walking yourself, but you know where you might find the most spectacular sights.
That said, I want to give some few examples that I found especially basic. Any one of these or similar articles will give you an understanding of what psychophysics is about. A good textbook should give you some references to more such experiments.
Charpentier on the Size-Weight-Illusion:
Charpentier, M. A. (1891). Analyse expérimentale de quelques éléments de la sensation de
poids. Archives de Physiologie Normale et Pathologique. Cinquième Série, 3, 122-135. Available online at http://gallica.bnf.fr/ark:/12148/bpt6k4327622/f125.image
An English translation by David J. Murray and Robert R. Ellis has fortunately been archived at http://www.oocities.org/robertellis600/charp.htm
Background information on Charpentier's article is provided in:
Murray, D. J., Ellis, R. R, Bandomir, C. A., & Ross, H. E. (1999). Charpentier (1891) on the size-weight illusion. Perception & Psychophysics, 61, 1681-1685. doi:10.3758/BF03213127
Plateau on Averaging of Achromatic Colors:
Plateau, J. (1872). Sur la mesure des sensations physiques, et sur la loi qui lie l'intensité de ces sensations à l'intensité de la cause excitante. Bulletins de L'Academie Royale des Sciences, des Lettres et des Beaux-Arts de Belgique, 2me Sér., 33, 376-388.
An English translation by Janet and Donald Laming has been published as:
Laming, J., & Laming, D. (1996). J. Plateau: On the measurement of physical sensations and on the law which links the intensity of these sensations to the intensity of the source. Psychological Research, 59, 134-144. doi:10.1007/BF01792434
The experiment has been replicated and discussed in:
Heller, J. (2001). Mittenbildung bei achromatischen Farben: Das klassische Experiment von Plateau. Zeitschrift für Experimentelle Psychologie, 48, 259-271. doi:10.1026//0949-39188.8.131.529
I'll update this answer with more examples when I have the time.