A fascinating topic. I've only read about the INA in South Asia, but a quick search threw up books you might find helpful: Jan Kuhlmann's "Netaji in Europe" (specifically on Bose's time in Europe) and Sugata Bose's "His Majesty’s Opponent". Many of Bose's own writings are also available in English, e.g. as "The Essential Writings of Netaji Subhas Chandra Bose" edited by Sisir K. Bose & Sugata Bose.

You might also find this talk by Satadru Sen from 1999 interesting, especially this part on Bose's European 'Indian army':

Bose's other major accomplishment in Germany was organizing the Indian Legion. In the course of the fighting in north Africa, the Germans had taken thousands of Indian POWs. Bose went to the POW camps, and asked the Indian soldiers to join him to fight against the British. This idea was not entirely new. An expatriate Indian named Mohammed lqbal Shedai had already made a start organizing Indian POWs captured by the Italians. For a while, Bose and Shedai ran parallel efforts with the POWs. Then the Indian soldiers in Shedai's camp mutinied, This discredited Shedai, and left Bose as the only Indian organizing an army to fight on the side of the Axis.

Ultimately, the Indian Legion that Bose organized in Germany numbered about 3000 soldiers. It was not easy putting this force together. Initially, when Bose visited the POW camps to talk to the soldiers, he got a hostile reception. The British Indian Army may have been an army without a nationalist ideology, but it would be a mistake to think of it as a purely mercenary force. The colonial army had its own ideology, which was a combination of loyalty to the British throne, loyalty to the commanding officer, loyalty to the regiment, and a tradition of service in which the same family or same village sent many generations of soldiers into the Army.

But Bose persisted. He had the men separated from their officers, and talked to them one on one. Eventually, he got through. He promised them proper treatment from the Germans, appropriate rank, pay and benefits, and he promised them victory. The British, he told them, were certain to lose the war; they were like a dead snake that people continue to fear. If only they shook off this fear, Bose said, victory and freedom would be theirs.

Bose had planned that when the German swing through the southern USSR and the Middle East reached India, the Indian Legion would spearhead the attack on India. He anticipated that this would have two results. One was that when the soldiers of the colonial Indian Army faced the Indian Legion, they would refuse to continue to fight for the British and desert en masse to Bose's side. The other was that an invasion of India by Indian Legion would galvanize the Indian population into rising up against the British, and make Bose's task easier.

As things turned out, however, the Indian Legion came to a rather sad end. First, the Russians stopped the German advance at the battle of Stalingrad. This, combined with the defeat at El Alamein, meant that the Germans would be unlikely to get anywhere near India. Second, after Bose left Germany in 1943, the Legion was left without an effective leader to look Out for them. They were absorbed into the German army and deployed in France. Now they knew they weren't going to be fighting for India's freedom, and their morale and discipline disintegrated. Many deserted, some joined the French resistance, and the rest disappeared in the chaos of the German retreat.

Bose's biggest frustration in Germany had to do with diplomatic recognition. He wanted Germany to officially recognize India as independent, and him as the leader of a government in exile. This the Germans refused to give him. The reasons lay partly in apathy, partly in the Master Race mentality, and partly in the peculiarities of Hitler's vision of the post-war world.

Hitler was not entirely comfortable with the idea of helping Indians - whom he saw as racially inferior - to defeat the British. The British were Aryans, after all. In his own way, Hitler admired the English, and through much of the war he continued to hope that he could come to some kind of an understanding with Britain: essentially, that Germany would get a free hand in Europe, and England would get to keep most of its empire. He was perfectly willing to use Bose to make trouble for the British, but he had no long-term interest in India's future, one way or another. Bose knew this, of course. He wanted to use the Germans for his own purposes. But eventually, as things turned out, neither was able to do very much for the other side.