why is it that 2 , 4 , 8 are compatible, is it because 2 , 4 , 8 are even numbers, but 3 , 6 ,9 are  compatible, because 6 may be a even number, but,   3 & 9 are not, even numbers, and why don't we count master numbers in  doing compatibly, test, is it because they are double digit numbers, & does anyone know where I can get a good book on  about numerology compatibly stuff.  

its a general rule numb  odd numbers carry a masculine vibration and reinforce each other as do even/ feminine  numbers .but you can also look at the numbers in other ways as in your question about the 3-6 9 combination all carry a creativity vibration   .

11/2 22/4 33/6  all even and  44/8       55/5 66/3 77/5 odd   most numerologist dont consider master numbers beyond 22  but there is a growing consideration that as the world involves into higher levels  that these other combinations will start to show there place as higher vibrations of the single digit they can be reduced to .

1 (Sun) and 4 (Uranus) are similar yang numbers. 2 (Moon) and 7 (Neptune) are similar yin numbers. 1 or 4 are compatible with 2 or 7. When you put the nine single numbers into a 3x3 grid, you get these three columns that supposedly reveal some comraderie: 1-4-7, 2-5-8, and 3-6-9. 5, being in the middle of the 3x3 grid, gets along well with the other numbers except for 8. Notice, that when you have Manifest and Soul numbers of 3 and 6, that you'll get the 9, which all three are in that right column of the 3x3 grid.

To answer your question about compatibility in Numerology it is a very completed one.

Many elements have to be looked at, it’s not a matter of odd or even numbers.
Some couples are together because their personality numbers are compatible, others have simular pinnacles or challenges for a certain time, and some are opposites, the strength of one is the weakness of the other so they learn from each other.

All aspects of the charts must be looked at and an overall evaluation and analysis of the couple can produce a wonderful guide to their lives. But it takes lots of experience and should not be attempted until you have much experience in Numerology.  
                                                     Kind regards  Wayne Rose
                                                         www.rosenumerology.bigpondhosting.com