On one hand, Tanaka Kakuei was about as corrupt as they come, and was infamous for getting his way through strategic - and mostly illegal - donations of cash. Nor was he averse to taking bribes. His involvement in the 1976 Lockheed Martin Scandal - in which Lockheed Martin bribed Japanese politicians to purchase the Lockheed L-1001 TriStar Airbus for All Nippon Airways - was just the tip of the iceberg when it came to Tanaka's dubious financial dealings. As the inheritor and owner of a large construction company, Tanaka had always mixed business and politics, and was not averse to using money to grease the wheels of government. According to the author, in 1957, when he was appointed as the unprecedentedly-young Minister of Posts and Telecommunication in the cabinet of Nobosuke Kishi, it was rumored that he had simply slipped Kishi a backpack (perhaps an odd choice of container?) filled with 3,000,000 yen.
He was also well-known for wining and dining bureaucrats to win their allegiance, and his business dealings were egregiously self-enriching. Tanaka invested his assets in a number of 'ghost companies' run by his own relatives, who would promise to oversee his money while he used internal, government information to promote their business (ironically, a practice that, until recently, was only partially illegal in the U.S...). His ascent to the Japanese premiership was also partly paved through money. In a dead-heat LDP intra-party election for Prime Minister against Fukuda Takeo, Tanaka supposedly slipped Nakasone Yasuhiro, the leader of a smaller faction and a future Prime Minister, an undisclosed sum of money to break the tie. Unsurprisingly, Tanaka won.
Of course, focusing on these aspects ignores Tanaka's considerable political prowess, popularity, and accomplishment. Tanaka was a remarkably effective legislator, and was re-elected by record margins even while imprisoned because of the care and attention that he lavished on his backwater Niigata district. One stat given in the book is that "In 1982 [he was still in office at this time], the residents of his prefecture paid an average of $541 in taxes and received per capita public works of $1,644. In contrast, Tokyo residents paid $3,060 for public works of $815." This was chiefly done through his expert administration of his koenkai, which are local groups used by politicians to raise funds, obtain support, etc. etc. Tanaka's koenkai was called the Etsuzankai ("Niigata Mountain Association"), which had offices and members in every town in the district. Etsuzankai networks would keep tabs on constituent requests, and Tanaka was especially careful to respond to their needs.
As Prime Minister, Tanaka's signature domestic achievement was to make controversial investments in Japan's infrastructure through deficit spending, which would fundamentally reshape Japan's landscape. On one hand, the bullet-train system was expanded and rural areas received an unprecedented amount of investment and industry, arguably providing the basis for Japan's economic rise. On the other hand, the system benefited legislators like Tanaka, who used their influence to locate high-profile - but superfluous - projects in their own districts.
Tanaka is perhaps best-known for his involvement in foreign affairs. He re-established diplomatic relations with the PRC in 1972, conducted some of the most constructive talks with the U.S.S.R. on the subject of the Kuril Islands in years, and was influential in opening access to oil and other resources throughout Southeast Asia. As the author notes, Tanaka also showed boldness during the 1973 Arab oil embargo, when he broke with the U.S. to side against Israel, so as to partly meet Arab demands and work to ease strains on Japan's oil supply.
On a final note, Tanaka was also a notable mentor and friend to future Japanese Prime Ministers and power-brokers. Nakasone Yasuhiro, one of Japan's most well-known and longest-serving Prime Ministers in the 1980's, obtained his position largely because of Tanaka's support (it was thereafter dubbed the 'Tanakasone' administration). In general, all of Japan's Prime Ministers throughout the late 1970's and 1980's needed at least tacit support by Tanaka in order to get elected. Nowadays, the infamous Ozawa Ichiro is a living embodiment of Tanaka's legacy. Ozawa was supported and shown favor by Tanaka early on in the 1970's, and although he broke with Tanaka in 1985, he now seems to be following in his mentor's footsteps by attempting to be a "shadow shogun" of the Democratic Party of Japan.
I should also mention that Tanaka, for all his corruption - and yes, womanizing - is still very popular in Japan today. His gruff style of politics is much admired, and in light of Japan's current generation of politicians, who are seen as action-averse and timid, his ability to 'get things done' is looked at wistfully. It might be similar to how, in the U.S., a small plurality of people remember Richard Nixon fondly, choosing to appreciate the overall character rather than to focus on his foibles and corruption.
All in all, an excellent read! Due to time constraints, I had to stick to this shorter overview of Tanaka, but I hope to read more about him in the future.