1. Conceptual and geometrical problems in quantum gravity;Isham;Imperial College preprint TP/90-91/14 London,1991
2. Quantum gravity: an introduction to some recent results
3. L. Smolin, Recent developments in nonperturbative quantum gravity, Syracus preprint SU-GP-92/2-2, to appear in Proc. 1991 GIFT Int. Seminar on Theoretical physics (World Scientific, Singapore, in press);
4. Narada's quest: Conceptual problems and present approaches to quantum gravity, talk delivered at the ICGC-91 Conference;Rovelli,1991
5. Superstring theory;Green,1988