I would like to know where I can get information on a Japanese man who
appeared on old 100 yen or 100 sen (I don't know which) notes.  I
believe his name is Easi Itagake, though I may have the name a little
wrong.  I would be interested in any websites or the names of any
books you would recommend that would have information about him.  If
you know where I could obtain such a book, I would like to know that
too.  Thank you.

Hi commedia:

Thanks for the great question. I, too, am very interested in monetary history. 

The gentleman on the old 100 yen note is Taisuke Itagaki. This
particular banknote was first issued on Dec. 1, 1953 and was suspended
on Aug. 1, 1974. However, it is still legal tender.

Source:
Valid Bank of Japan Notes
http://www.boj.or.jp/en/money/basic/now/data/money01.pdf
(scroll to near the bottom to see a picture)

As far as information about Taisuke Itagaki, I was able to find the
following websites with some information about why he was important
enough to be on the currency:

Itagaki, Taisuke
http://www.infoplease.com/ce6/people/A0825663.html

Chapter 7: Colonial Modernization
http://members.shaw.ca/leksu/mainp7e.htm

Count Itagaki Taisuke 
http://www.britannica.com/eb/article?tocId=9043010

Meiji restoration
http://www.infoplease.com/ce6/history/A0832538.html

While I wasn't able to find any books (in print, in English)
specifically about Taisuke Itagaki, I was able to find a couple of
books that list him prominently in their indexes:

Sakamoto Ryoma and the Meiji Restoration
http://www.amazon.com/exec/obidos/tg/detail/-/0231101732/103-6718938-1242209?v=glance
Index: http://www.amazon.com/gp/reader/0231101732/ref=sib_dp_bod_idx/103-6718938-1242209?%5Fencoding=UTF8&p=S0CA#reader-page
(second page in)

The Emergence of Meiji Japan (Cambridge History of Japan S.)
http://www.amazon.com/exec/obidos/ASIN/0521484057/qid=1109609502/sr=2-5/ref=pd_bbs_b_2_5/103-6718938-1242209
Index: http://www.amazon.com/gp/reader/0521484057/ref=sib_dp_bod_idx/103-6718938-1242209?%5Fencoding=UTF8&p=S0A2#reader-page
(third page in)

There are also three books listed on Amazon as specifically about Taisuke Itagaki:

Amazon Search
http://www.amazon.com/exec/obidos/search-handle-url/ref=dp_srsubj_entry/103-6718938-1242209?%5Fencoding=UTF8&index=books&field-keywords=Itagaki%2C%20Taisuke

However, they all appear to be in Japanese and all are out of print. 

Search Strategy 
(on Google):
* Japanese currency
* Taisuke Itagaki
* Taisuke Itagaki book
* Meiji Restoration
(on Amazon):
* meiji restoration 

I hope this information helps you out. 

Thank you again for the question. 

websearcher

Excellent answer.

Hi commedia:

Thank you for the generous rating and tip! Glad I could be of help. 

websearcher