According to the book, he built that car in 1951 and was powered by a 350cc motorcycle engine. Looking at the pic (that is not in the book) the description seems to fit. There is not a base car there...I won't be surprised if it is based on a pedal car.
In view of Arunas's remarks perhaps I should give a point to the one who has posted most information so far, who is Wendax.
I still won't call this puzzle solved however as there is one question still to be answered, which can be by anyone.
This is the information I have about this car, most of which has already been given here in replies:
"This small car is a one-off built by Stig Ihlberg in Sillerund, Sweden 1948-49.
It was very popular in Sweden at that time to build from parts left in the yard. HPD is short for in Swedish "Hop-Plockade-Delar" which translates as "Gathered picked parts". All these types of cars got the name, when registered for road, HPD and a number. This car got the registered identification HPD Special ***** *****. It had a two-stroke 19 PS engine and weighed 440 kg. The car was scrapped in 1957. It had a pipe frame and on this a welded plate chassis. The plates were taken from a *****."
Who can convert the *s into the missing words? Points are there for the taking!