What does the "L." stands for
example: Ceratonia siliqua L.

Linnaeus. Carolus Linnaeus or Karl von Linne was the father of our modern
binomial system of scientific names. Any botanical name with L. after it dates
from 1753. If you want to pay your respects, there is a heroic size statue of
the gentleman in the Chicago Botanic Garden.

Are there other Letters like these used?
yes. In the full formal citation of a scientific name, the genus & species are
followed by the "author" or "authority," the person who published the name.
They are usually abbreviated.
There are a great many. I could name some, but you might get indigestion. Some
of the most important in orchids are Lindley, Reichenbach jr. (abbreviated
Rchb. f.), Rolfe, Kunth, and today Averyonov, Christenson, Gruss, and others.
If you are mainly a gardener, you don't have to worry about them.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)