I did a quick but targeted search in my library for Fiat 850 derivates and the following list is what I came up with. But i'm sure there are still (a lot of) cars missing:
A lot of them are already mentioned above but some are new in this thread.
Let me know from which ones you need more information. (Although most of the time not a lot (sometimes even none) of additional info is available). Please keep also in mind that sometimes a car in this list and a car that is already mentioned above can be the same car but with a (slightly) other name or description:
1964 Abarth OT 850/130 Berlina (standard body)
1964 Abarth OT 1000 Berlina (standard body)
1964 Abarth OT 1600 Berlina (standard body)
1964 Abarth OT 2000 Berlina (standard body)
1965 Abarth OT 1000 coupé and OTS 1000 S coupé (standard body)
1966 Abarth OT 1300/124 coupé (standard body)
1966 Abarth OT 2000 coupé
1968 Abarth OTS 1000 Coupé gruppo 3 (standard body)
1965 Allemano coupé Fiat 850
196? Allemano Coupé
1965 Allemano Berlina 4 porte
1972 Baldy Elsabry
19?? Baldy Riky
1965 Bertone Spider by Giugiaro (normal body)
1965 Bertone Fiat Abarth 1000 OTR 1000 coupé by giugiaro
1965 Bertone Fiat/Abarth OT 1000 spider (standard body)
1968 Bertone Fiat 850 Racer Team
1964 Boneschi Daino
1964 Caprera Fiat 850 Belvedere
1967 Caprera Fiat 850 Elaborazione (sun roof and improved interior)
1966 Caprera Fiat 850 Coupé
1964 Ellena Fiat 850 Spider with both Abarth 1000 and Fiat 850 mechanics
1968 Eurostyle Fiat 850 coupé (2 different ones)
1964 Fiat 850 (Standard car)
1965 Fiat 850 Coupé (Standard car)
1965 Fiat 850 Spider (Standard car)
1968 City-Taxi
1967 Francis Lombardi Fiat 850 Special (with big sunroof)
1968 Francis Lombardi Fiat 850 ‘Smart’ on both 850 and 850 special basis
1964 Francis Lombardi Fiat 850 1st series “Lucciola” 4 doors (3 side windows)
1964 Francis Lombardi Fiat 850 “Lucciola” 4 door (2 side windows)
1964 Francis Lombardi Fiat 850 and 850 Special “Lucciola” with simili-leather interior
1964 Francis Lombardi Fiat 850 “Libellula”
1969 Francis Lombardi Grand Prix spider (also called Monza?)
1968 Francis Lombardi Grand Prix coupé ( Giannini 850 and Giannini 1000 [not identical] and Abarth 1300 Scorpione and Scorpione S and SS)
1968 Francis Lombardi Special Grand Prix
1968 Francis Lombardi Monza
1965 Frua Fiat 850 (modified rear window)
1964 Giannini 850 S and 950 (standard body)
1965 Giannini 850 SL (standard body)
1965 Giannini 850 Coupé and 950 and 1000 (standard body)
1965 Giannini 850 Spider and 950 and 1000 (standard body)
1968 Giannini 1000 Berlina (standard body)
1964 Ghia Fiat Vanessa by Giugiaro
1966 Ghia Roots (only a drawing and not sure it is Fiat 850 based)
1963??? Mantelli elaborazione 850 SS
1964 Mantelli Fiat 850 4 door by Michelotti
1964 Moretti berlina 4 door
1964 Moretti coupé Fiat 850 Special
1964 Moretti Spider Fiat 850 Special
1966 Moretti coupé 850 (sportiva model)
1966 Moretti coupé 850 SS (sportiva model)
1966 Moretti coupé 1000 SC (sportiva model)
1966 Moretti 850 Sport Competizione (sportiva model with T-roof and Giannini engine)
1967 Moretti 850 (sportiva model)
1968 Moretti Berlinetta Sportiva on Fiat 850 Special (sportiva model)
1968 Moretti Coupé on Fiat 850 Special (sportiva model with sun roof)
1969 Moretti Berlinetta Sportiva on Fiat 850 Special (sportiva model with other front)
1968 Michelotti Shellette on Fiat 850 Special basis
1968 Michelotti Coupé Fiat 850
1967 OSI Week-end
1964 OSI Coupé by Michelotti
1965 Pininfarina Fiat Abarth 1000 Coupé Speciale (the Spider has 600 Mechanics)
1964 Savio Spider
1964 Scioneri Coupé by Michelotti
1964 Scioneri Spider by Michelotti
1964 Scioneri Coupé by Barison
1966 Scioneri 850 elaborazione
1964 Siata coupé TS Fiat 850 by Michelotti
1967 Siata Spring
1964 Touring OSCA 1050 Spider
1964 Vignale Coupé 4 seats (Also as Giannini 850 S and 950)
1964 Vignale coupé 2+2 (Also as Giannini 850 S and 950)
1964 Vignale Spider (Also as Giannini 850 S and 950)
1969 Vignale “850 Special Export” in I think both 2+2 and 4 seat
1965 Vignale Berlinetta coupé
1969 Vignale Dart on Fiat 850 Special basis
1964 Zagato Fiat 850 coupé Z
Because of the complexity of the Giannini and Abarth line-up some more in dept research will be necessary.
There are also the ‘license built’ Fiat 850 cars like the Neckar Adria, Pirin-Fiat 850 and Seat 850 and its derivates.
850 T derivates are not mentioned in the list because you already published a book about these.
I don’t know how far you want go with your book but cars like the Autobianchi A 112 had Fiat 850 (derived) engines as well.