"He was steeped in a to many people boring book?

That very particular sentence is very hard to read, so most people would say it is 'ungrammatical'.

However, the pattern that it seems to allude, a not uncommon one, while not formal (and so ungrammatical or rather unacceptable in a newspaper or formal speech) is to use an extended noun phrase syntactically like a preposed noun-attribute, modifying the following head noun.

To modify your example a tiny bit:

He was reading a boring-to-too-many-people book.

This is very informal and would not make it past an editor or school teacher and called horribly ungrammatical. That said, it is a pattern that many native speakers use very informally (the way to say this is that the pattern is 'grammatical in a particular informal register').

The sentence would be translated to a more formal register as

He was reading a book that was boring to too many people.

As with lots of informalities like this, if you are a non-native speaker, I would avoid using this kind of pattern because it will sound like a mistake if said with an accent. For a native speaker, it should probably be avoided in more than informal circumstances.