If robot has parallel joint axes,the D-H parameters are not unique.So what can we do to deal with this problem? It is very nice for you to answer or recommend some paper and books. Thank you~
This is an overview of the DH Convention, there is a piece where they discuss having parallel joint axis.
IMO I believe that it is best to choose your parameters for that axis in the best way to get to the next joint, or one that represents the physical bodies best. If you choose a solution that makes it difficult to make the next set of parameters, you're causing headache for yourself in the future. Let me know if that helps.