@Zephyr with respect to your second question there are several excellent texts available, really depends on your field. You might consider the Econometric Analysis of Cross Section and Panel Data, by Jeffrey Wooldridge. Additionally, you should look at text SAS for Mixed Models. Both textbooks have second editions. Your first question is a bit confusing in that you refer to fixed effect mixed models, the idea of a mixed model is that it has both fixed and random effects, hence mixed models. I am sure the texts mentioned will help you understand how unbalanced data are handled in the mixed model framework. I hope this is helpful, MG.