in Germany, I would recommend to go to the Frankfurter Book fair in autumn this year, I know there is one publisher, who likes to see only whole book concept, I cannot remember its name (will ask my colleagues). But other publishers have also free portfolio hours, where you can show your work and/or your portfolio.
Other thing, many publishers are offen strict with their text and illustrations in Gemany, many of them can like your text as author, but want to chose their own illustrator for the work.
I don't know how far the free agents work with finished books, I know only of taking care of their illustrators and finding them jobs for illustrating books for publishers. I don't know how it works for authors.
But all the big publishers have contact site on their website, where there is this contact for authors/illustrators, where they can send their manuscripts and illustrations. After that the editors look at the work and contact you if they are interessted.
What I would send are small thumbnails, Cover, contents and 3 example sites as PDF or printed version, if you have only shipping adress.
Hope I could help You some.