Mechanics of Spaceflight Low-Thrust
By G. L. Grodzovskii, Yu. N. Ivanov, and V. V. Tokarev. Translated by A. Baruch. Israel program for Scientific Translations, Jerusalem, 1969. NASA Technical Translation F 507. 
Includes sections on solving for optimized trajectories for solar sails and radioactive isotope sails. This section is on pages 315-325.
- Grodzovskii, G L and Ivanov, Yu N and Tokarev, V V. Mechanics of Spaceflight Low-Thrust. Israel program for Scientific Translations, Jerusalem, Israel, 1969. BibTeX