It's only been a few months since I found that there were three sequels to Paul Berna's The Horse Without a Head (later made into a amazingly faithful Disney adaptation). In early April I bought what I thought were the second and third book in the series on e-Bay. Surfing about Amazon.co.uk last week I found the fourth book, Gaby and the New Money Fraud, at the low price of £2 (an amazingly low price since the only other copies I could find were going for between $44 and $131 on Amazon Marketplace and £9 and £106 otherwhere on Amazon.co.ukno cheaper than $18 on bookfinder.com!). At first it looked like the vendor didn't ship outside the U.S., although the notation said "International Delivery available," so I e-mailed the person a note and they said they would ship to the States and did whatever they had to do at their end so I could order.
Well, it has arrived here within eight days! Not only that, but a bigger surprise: it's actually the third book in the series. The copyright date is down as 1971, but it was published in 1961 in France. It explains how the kids got the van that they drive in Mystery of Saint-Salgue and a mystery surrounding it. Inspector Sinet also appears in the story, although he was missing from The Street Musician.
(Postscript: It's been very enlightening to read this book "after the fact," as it also provides the setup for Fernand's destination.)