There are actually a few interpretations of the Wittig rxn mechanism, depending on the solvent, and some special mechanisms have also been proposed. There are also arguements regarding whether the betaine actually exists in solution, and NMR studies have been performed to determine that. A simplified idea of the mechanism can be found in Clayden Wothers. However i suggest you look up the following papers:
Bestmann, Pure Appl. Chem. 52, 771-788 (1980)
Speziale, Bissing, JACS ,85, 3878 (1963)
Olah, Kâmurthy JACS, 104, 3987 (1982)
Allean, Heatley, Hutley, Mellor, J. Chem. Soc. Perkin rans 1, 2529 (1976
There are other papers too, but i found these particularly interesting.
This is actually one of my, um, favourite reactions. If you are really interested, check out the following papers. i found them very interesting:
Ando, J. Org. Chem., 62, 1934-1939 (1997)
Blanchette, Choy, Davis, Essenfeld, Masamune, Roush, Sakai, Tetrahedron Lett., 25, 2183-2186 (1984)
Schlosser, Christmann, Piskala, Coffinet, Synthesis, 29 (1971)
Boden, Synthesis, 784 (1975)