The Japanese Military and government were divided into two different camps prior to the war. One contingent thought that their future lie in the east, and the other contingent thought that their future lied to the south.
Japan didn't have the resources to go in both directions.
The issue was effectively settled for two reasons:
in 1939 Japan lost at the Battles of Khalkhin Gol. Not only were the Soviets stronger than the Japanese had thought, the Soviets also deployed large numbers of tanks and artillery.. things which the Japanese lacked.
Germany didn't declare war against Russia until June of 1941. Japan had already decided to attack in the south/south east in 1940 when the allies were at their weakest. And remember, they could do one or the other, not both. So by the time Germany was at war with Russia there was really nothing that Japan could do to help.
In my opinion there are really not a lot of good books on Japan in World War 2. As far at the Japanese are concerned the war started for them in the early 1930's. If you pick up a book that starts basically at pearl harbor, you know it's not giving you a good answer.
My above answer comes from what I think is the best book on the war form the Japanese perspective:
(Dear mods, I haven't posted a top level comment in two years. I hope this is ok, I just read the above book and I think my answer is sourced and on point).