I would like some recommendation for resources that will help me understand physical, social and mental development in a child. Also, it would be great to have some resources that suggest activities/topics/etc. that would help the child develop at different ages.
I would say that the best resource (INHO) is your pediatrician. Honestly it is their business to know what your child's needs are.
I also found Baby Center to be a great reference with regular email updates and a nice community.
Then there is the classic "What to expect when..." series of books.
In addition to speaking with your pediatrician when you have a specific question, I liked the "The What to expect" books for the most part, but I actually found Dr. Sears' Book, "The Baby Book" more useful (especially in regard to suggestions etc.) during my daughter's early years. Here is a link to a related website with contributors that are all pediatric physicians or nurses.
I have also thoroughly enjoyed a book called "The Busy Book" with all kinds of ideas for activities to do with your toddler. I also offer up ideas from my daughter's own toddlerhood, time with a little boy I nanny, and memories from my years as a preschool teacher on my website for activities that are particularly fun from time to time.