I think that Pa Salt is in all of the books. He was in his late 80s in 2007, and the little French boy in the first book was found in 1928 at 7-8 years old, he plays Morning Mood, he is intrigued by the stars, and Isabella gives him her Brazil address.
In the next book, Bo resembles a lot of Pa Salt's qualities, and again is an accomplished musician. We don't know much about him, but his girlfriend is a Parisian orphan. Towards the end, he starts working for a map maker, and again, shows a strong interest in boats, astronomy, and the Seven Sisters waterfall.
In the next book, Mr Tanit doesn't get much of an introduction, but he again has some Pa Salt-like qualities. Also, Tanit is an anagram for Titan - which is also the name of Pa Salt's boat, and Atlas, the greek god Pa Salt is named after (Pleiades ATLAS), was a Titan.
Not sure about the next book - James on the boat popped up about the right time, but he didn't have the same mysteriousness of Pa Salt's other appearances. There were no other likely candidates in my view though.
In the Moon Sister, he is the man who bought Tiggy's mother an ice-cream and had his fortune read by Angelina. He was a kind man, and the mysterious mention of his daughter linked it directly back to his letter to Tiggy at the start.