I just found a way to import the paper book I am reading, it is not too laborious, but I am kinda amused by how I managed to make it work.

1. take pictures of book pages with my phone, one chapter at a time
2. select them in gallery in reverse order, share to google keep as one note.
3. in the browser version of Google Keep use function 'grab image text'
4. delete images from the note
5. export to google docs
6. remove line breaks with a help from an add-on
7. save as txt
8. drop it to Lingq as a new lesson