Are geminal disubstituted alkenes more stable than their cis/trans isomers?
This is a very good question and if popular books give conflicting answers, then it must be reasoned out. Unfortunately, Paula Bruice has given the wrong answer while the other two books have given no explanation for this comparison.

For the answer I assume that you know about hyperconjugation and the various contributing structures it involves.  This gives a basic idea of hyperconjugation:  


Source: tutorcircle.com  

Note that on hyperconjugation, the other carbon of the alkene gets a negative charge on it.

If the hyperconjugation structures of 2-methylprop-1-ene and trans-but-2-ene are drawn:




you will notice that:


$ \ce{C_1} $ of the former has a negative charge on it 
$ \ce{C_2} $ of the latter has a negative charge on it 
The latter's methyl group makes the negatve charge less stable due to +I (inductive effect) 
The former has no such destablizing effect as the carbon containing the negative charge is just connected to hydrogens
As all the hyperconjugation structure have this effect applying on them, the net effect cant be ignored for the overall stability 


Hence the overall stability of  2-methylprop-1-ene > trans-but-2-ene