Conserning explanations about the twin paradox (John Mallinckrodt gave two ways to explain it).
Swartz & Miner (1997, 145-148) provide a detailed quantitative explanation in terms of changing the reference frames in the context of Special Relativity. This is an elaboration of the other explanation pointed out by John M. I would say that Swartz & Miner's book belongs just next to Arons' book in a physics teacher's bookshelf. The physics is excellent (as far as I can tell).
A third kind of explanation (Linton 1997) utilizes time dilatation and Doppler shift. It is written in a way that it can be understood by a clever high school student.
Sometimes it is stated that General Relativity is inevitably needed in the analysis of the twin paradox. Hestenes (1990) discusses how this misconception was initiated by Einstein himself! Hestenes goes on and states that "he may have harbored some misconceptions of his own relativity". Interesting indeed.
Hestenes, D. (1990). Secrets of Genius. New Ideas in Psychol, Vol. 8. No2., pp. 231-246. Available online at < modeling.la.asu.edu/R&E/SecretsGenius.pdf>
Linton, J.O. (1997). The twin paradox explained. Physics Education, 32, No 5.
Swartz, C. and Miner, T. (1998). Teaching Introductory Physics - A Sourcebook.
Springer-Verlag, New York. ISBN 1-56396-320-5
This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.