First, to get this out of the way: Since Buch is neutral, it's ein überschätztes Buch, not ein überschätzter Buch. Even after this correction, only two variants are correct, idiomatic ways to call a book overrated in German:
- "FSoG wird überschätzt." (This is the normal idiomatic way of saying it.)
- (dubious) "FSoG ist ein überschätztes Buch." (Definitely not the normal way of saying it, but I guess it's possible if you absolutely want to use überschätzt attributively.)
The reason for this, in a single sentence, is as follows: The German point of view on this is that people overrate a book, but being overrated is not an inherent quality of the book. Read on if you are interested in the gory details.
Whereas the attributive use of überschätzt is just unidiomatic and slightly weird, it is definitely wrong to put it as a stative passive:
- (wrong) "FSoG ist überschätzt."
(In German, that is. In English the boundaries between dynamic and stative passive aren't as clear because they are formed identically.)
This is wrong because it is blocked by the first variant.
Finally, the last remaining variant is based on the already dubious second one, but says something else:
- (dubious; different meaning) "FSoG wird ein überschätztes Buch."
To the limited extent that anyone would say that, it would mean: "FSoG is becoming an overrated book."
We can verify what I wrote in Google's corpus of digitalised German books. (Note that I formed my opinion first. I didn't fit it to the result of a possibly flawed experiment.)
Here you can see that the stative passive wird überschätzt (sentence 1 in my ordering) is at least five times as common as ist überschätzt. The latter could be a dynamic passive (sentence 3), but as you can see, nowadays the present perfect stative passive ist überschätzt worden accounts for most uses. These should really be subtracted from the second n-gram. When fed correctly, the n-gram viewer does this for us. (We shouldn't add ist überschätzt worden to wird überschätzt because then we would also have to add ist überschätzt gewesen to ist überschätzt to be fair. We'll look at these separately below.)
Finally, the n-gram viewer can also divide the top curve by the bottom curve to give us usage ratios: final result. As you can see, the dynamic passive construction is currently about 10 times as common as the stative passive and the predicative adjectival use that happens to look the same.
What is interesting, however, is the fact that this appears to be a relatively recent phenomenon. Clearly, calling books overrated is a relatively recent idiom, at least in German. Apparently the way it is used is still sharpening, though it also looks like there were some setbacks. My guess is that these are due to distortions caused mainly by poor translations from English.
[Edited: I just realised that I promised something above and never did it. Here it is.]
Let's also compare the frequencies of the present perfects of "wird überschätzt" and "ist überschätzt". They are "ist überschätzt worden" and "ist überschätzt gewesen", respectively. The advantage of comparing these is that unlike "ist überschätzt", neither can occur in any unintended grammatical construction. The disadvantage is that it's not a very natural tense for calling a book overrated. The result could hardly be clearer: "ist überschätzt gewesen" doesn't even exist in the corpus!
Looking at the counter-examples
A quick scan through Google Books hits for "ist überschätzt" confirms this. The majority of hits is actually irrelevant due to a comma that is ignored by Google Books search but not by the n-gram viewer: "ist, überschätzt". As we already know, the remainder consists mostly of instances of "ist überschätzt worden".
Much more interesting are the few genuine cases of "ist überschätzt". I'll list the first few (for me right now):
- "KONSENS IST ÜBERSCHÄTZT" - a chapter heading in a 2011 cheaply produced motivational/self-help book. Most likely a failed translation of "consensus is overrated".
- "Die Lautung ist überschätzt, insofern sie nicht das Wesen der Sprache ausmacht [...]". - From an article by someone with a Slavic name in a 2007 collection of texts about Heidegger. Academic texts are often produced by non-native speakers and very rarely receive proper editing.
- "das ist echt. .. ich finde das ist überschätzt". - This is interesting because it is an example of colloquial speech that was used for academic purposes and therefore didn't receive the normal (for German editing - English editors don't do this) grammar correction treatment. From a 2014 book on start-ups.
- "Kontrolle ist überschätzt, Zutrauen unterschätzt." - A quotation ascribed to the founder/owner of a German chain of chemist's shops in a 2013 book on company philosophies. Most likely a mis-translation of "control is overrated", possibly produced in the heat of a discussion.
After this there were several pages without any examples, so I gave up. The important thing is that whereas the other hits (which involved commas, periods, semicolons or the word worden) included many examples of good prose, the above four can all easily be explained as grammatical errors. However, there is a pattern that suggests that in German business lingo, which often features weird Americanisms, the stative passive construction with überschätzt could actually be considered grammatical. The caveat here is that this kind of business lingo is generally considered ungrammatical by editors and the wider population.