> The sound change -hon to -ppon and similar sound changes are not actually regular in Japanese -- there's not a clear phonological rule for them. There is a l

> The sound change -hon to -ppon and similar sound changes are not actually regular in Japanese -- there's not a clear phonological rule for them. There is a loose rule for compound word contractions (adding tenten to the first letter of the second word), e.g. "sakura" gains a tenten in "senbonzakura". However, as you said, this is not universally the case. For example, "ehon" (lit. drawing book) is simply "e" (drawing) + "hon" (book), not "ebon".