Sanskrit character sukashi in a large iron tsuba.
The rim is carved from the plate rather than raised or attached. This is late Edo period work, but quite well done. It keeps some of the spirit of early katchushi tsuba, but takes it to an entirely more elaborate end. Nice that hitsuana were not included.
At first glance it looks quite abstract, but it’s not hard to figure out. The bonji at top and bottom are turned ninety degrees. Looking up the particulars of the Buddhist deities named is as they used to say, left as an exercise for the student.
Tsuba Kansho 
Is there a book describing Bonji?
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John Stevens’ “Sacred Calligraphy of the East” has a section on Siddham that shows the usually encountered bonji. It looks like it’s still in print and reasonably priced. There are more books in Japanese on the subject, but I don’t have any experience with them.
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There is a bonji Taikan, however unless you know Sanskrit or what you are looking for it is difficult. Complete language and many thousands of bonji…which look alike, but are different.
SO going to the one which show the most often encountered is probably best.
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