(The domain http://www.wasan-bibliography.com/ now links to this webpage.)
I have created a bibliography on traditional mathematics in Japan (wasan) and its historiography. It is limited to works in Western languages. While there are many more works in the Japanese language, consulting the works listed should already help to get a good grasp on the topic. It can also serve as a hub for researchers to know about each other’s work, which is largely scattered over a variety of journals and edited volumes.
You can find at: https://www.zotero.org/groups/2161796/math-in-japan/items
(While I recommend browsing the Zotero library because of the tagging and the better searchability, a PDF file reflecting the contents of the library on January 8th 2020 can be found here.)
For ca. 1600-1730, the period culminating in the classical results of Seki Takakazu and Takebe Katahiro, the best monograph in a Western language is the French book [Horiuchi 1994]. Recently, an English translation [Horiuchi 2010] was published. The new standalone publication [Sakurai 2018] is, as other works, filled with (self)-orientalisms, so while it can serve as an exposition, it should not be cited in scholarly works without caution.
[Mikami 1913] and [Smith and Mikami 1914] are outdated and should be read together with more modern articles. On the introduction of Western mathematics via Chinese books into Japan, consult [Kobayashi 2002] and [Kobayashi 2013]. Although it is generally not referred to as wasan, literature about the mathematics in ancient and medieval Japan (which [Jochi 2000] refers to as the mathematics of the “ritsuryō-kakushiki age”), is included.
A bibliography on traditional mathematics in Japan in Western languages until 1999 can be found in [Ogawa 2001]. The works listed there will show up here bit by bit. Another bibliography is [Heeffer 2006].
I plan to expand it with time. If you know works that appeared since the year 2000, please drop me a mail at harald[at]hkuemmerle.de!
Literature cited on this page (much more can be found on the Zotero site linked above):
For my own publications (which concern the history of modern mathematics in Japan) see my profile at academia.edu.