I have read the book Custody (YHM is inspired). Please note I am not making comparisons. Just information...
a) In the book, Shagun and Ishita do not meet till the end. Even then for a few minutes.
b) Ashok Khanna's character is better etched in the book. He is shown to be classy and dignified. No enmity towards Raman. He feels bad for taking Shagun but cannot help his feelings
c) No Parmeet or the likes. This character needs to be taken out in the serial.
d) Ishita is a teacher and not a dentist.
e) Ashok Khanna falls in love with Shagun and marries her in the book. Just does not lust for her as shown in the serial.
f) Shagun has a mother who supports Raman (not sure if she will make an entry in the serial).
g) Focus is on the custody and its effect on children and RKB and Ishita story. In the serial, true EK style, it is all over.
I love the Ishra story and hope the serial focus is on them and on Parmeet and the likes.