If a pregnant women reads or studies suppose English or Mathematics or Science books during her trimesters, then does it really helps in brain development of the child ?
No. This is pure myth. But it might make your wife/partner smarter :-)
What does help brain development is a sufficent and well-balanced diet (well-balanced for pregnant women, not for someone who wants to lose weight! Your wife/partner should listen to her body to tell her what she needs), NO alcohol, NO smoking, not even passive smoking (this doesn't help brain development, the point is not to damage brain development). Certain vitamins and minerals are also very important for the normal development of the child; there are special products designed for pregnant women. Your wife/partner can ask her doctor about them, or about any other dietary questions.
I've heard it said that lots of fish is good for brain development because of the omega 3 fatty acids, but I don't know if this is really supported by evidence. For what it's worth, we've got a few healthy, pretty smart kids without my wife gorging herself on fish during pregnancy. If I were you, I'd be doubtful of any advice that put too much importance on a specific cause for brain development. Also, even if there is a way to positively influence brain development in embryos, it will probably turn out to be almost insignificant. Don't forget that a huge amount of brain development happens once the baby is actually born, and brain development isn't nearly finished until the late teens. So this is where you should concentrate your efforts - give your baby and toddler lots of love and attention once it's born, read to her, play with him - this will all dwarf anything you do now (while it's in uterus).
The important thing now is not to do anything that will cause lasting defects (such as drinking and smoking, or trying to lose weight during a pregnancy without first talking to your doctor, or getting the mother infected with a variety of diseases that hurt the child).