Added Dec.'03: I also just noticed a typographical error on page 39, equation (85): the numerator of the right-hand side contains a factor 207^{2}, which cannot be right (207 isn't prime); it should be 307^{2}.
...and March '04: page 40, line 4: may fails --> may fail
...and September '04: pages 19, 20, 21, the underlined headings l=5, l=7, l=13 should have l in math mode. Less trivially, John Voight notes the following two corrections. On page 9, in the first line of text below formula (19), the factor (1-zeta)^{gamma} should be zeta^{1-gamma}. On page 41, the example l=73 cannot be right because it yields a curve of genus 2, not 1. I should have asked these questions for l=37, the largest genus-1 case (see the table on page 18); there the map has degree 38, still intractably large, and here Cremona's tables list nine curves of conductor 6l, in five isogeny classes (whereas for l=73 the counts are twenty -- not six, as I inexplicably wrote -- and seven respectively).
...and November '04: page 26, line -2, the opening parenthesis in ``Gamma(1)'' should not be an exponent [sic(k)].