Write Now is my favourite handwriting book. It includes very explicit instruction on making strokes, forming each letter, and variations in shape, slope, size, spacing and speed to suit your personal comfort. It also shows modifications and alternative ways of writing some of the letters.
The authors - Barbara Getty and Inga Dubay - conduct handwriting improvement workshops for doctors. The book includes several before/after handwriting samples of MDs and other adults, which might be helpful to older students who are remediating their handwriting.