Hi Yuuzan. Welcome to the forum. I trust that you will find much of interest here in regards to keris and look forward to your participation. In many ways asking questions is as important as knowing the answers.
As far as you first question is concerned, i don't have any personal experience with any books on this subject specific to Jawa. I believe part of the problem is that most of the books available in the English language were written from the perspective of European colonialists who didn't really gain a foothold in Jawa until the time of the Islamic conversion. Probably the best place to look then for information in regards to Hindu/Buddhist influences would be books on Bali where those influences remained in power. The first book that comes to my mind that you might want to read in Margaret Wiener's
Visible and Invisible Realms: Power, Magic and Colonial Conquest in Bali.
I have also found Fred Eiseman's two volume
Bali: Sekala & Niskala a valuable read.
