Too many questions but I will try to answer them,
Subhadra is one of the most important characters when one considers Mahabharat. Her grandson Parikshit became the heir to the Kuru Rashtra after Yudhisthir's reign. All the sons of Draupadi, Shubhadra, Uloopi and Hidamba died in the great war of Mahabharat. Abhimanyu, Subhadra and Arjun's son was a great warrior in a very young age and hence importance is given to Subhadra for giving birth to Abhimanyu.
Your second question doesn't make much sense, why is she ignored - What makes you feel she is being ignored and by whom?
Rath Yatra should give you insight that she is given equal position and importance when it comes to worship and Krishna.
Not sure if there is any book on her, I haven't come across any. But reading this article should give you more insight (if you already haven't)
Not entirely sure about favourite wife but from what I have read at various places - Uloopi comes up to my mind. She has releaved Arjun from curse, brought him back to life, gave him a boon, sacrificed her son Iravan for the Mahabharat war and many other reasons.
And without a doubt the main and first wife of Arjun was Draupadi.
Bonding between all of these can be obtained by reading the Mahabharat and also the Bhagwat (There are variations of Bhagwat) Selecting specific chapters from Mahabharat will help you understand the various situations and bonding between these characters.