David Sear's book has some information on legends. The
DOC books, which are now available online, have a lot more information. Grierson's single volume book
Byzantine Coins has some information, but this is quite limited.
Legends changed quite a lot during the
Byzantine period. Legends were more standardized at first, but the standardization and
consistency broke down towards the end. Coins from the Paleologan period are quite variable for example. The
legend might change from die to die, with blundered legends and missing letters.