You might be interested in this book:
I read it. It certainly did not not convince me LBJ had any role in the assassination. However I highly recommend people read it for another reason - to get insights into how much of an SOB LBJ was. For example, LBJ wasted no time in basically throwing Jacqueline Kennedy out of the WH which was pretty tacky. Plus the cover photo shows LBJ was such a sneaky look on his face that it almost makes one wonder though.