Since there are more and more web novels posted here I wonder if there also good hentai novels?
I searched for a bit but I couldn't find anything.

I don't know if it's considered hentai, but you might take a look at Masou Gakuen HxH (Novel). It's pretty darn extreme. It has numerous scenes of the main character "healing" his comrades in arms by bringing them to climax after climax, with thorough descriptions of the process, as well as a scene of him transforming a seventh grader into a soldier by inserting a device into her that spontaneously took on his shape and transferred sensation to him, and much later a scene where he pleasures and is pleasured by a pair of princesses while all his female comrades and classmates are forced into a massive orgy of uncontrollable lust. I stopped at that point, it was too much for me. It's not pure hentai, there's a lot of combat and conflict and relationships, but still when those scenes come along they are quite detailed.

https://lightnovelstranslations.com/himekishi-ga-classmate-isekai-cheat-de-dorei-ka-harem

Well, this one should be pretty good, I think? The hentai was so-so, but the story itself is pretty funny for me.

And yes, this one is full blown hentai.

Not sure if you're still interested in this, but seeing how few titles were mentioned here, I thought I would name few more.

Shinmai Maou no Testament (Novel) - is not a hentai, but it does include quite a bit of erotica. The scenes are not as detailed as in H-novels though.
Maou no Hajimekata (Novel) - also not a hentai, but includes numerous sex scenes and they are very detailed.
Shishunki na Adam (Novel) - didn't read this one yet, but apparently it is fairly close to a hentai.

here are a few i know of:

• Isekai Taneuma
• Flirting With Beast Girls! Doing Nothing but Copulation!
• Pure Love ✕ Insult Complex
• Asuka of the Scarlet Sky
• Dragged Into the Class Metastasis for Some Reason I Was Dragged Into the Metastasis With the Girl Class so I Will Make a Harem!
• I Made a Slave Harem Using a Charm Cheat in a Different World
• Because I Was Excluded Out of the Class Transfer, I Decided to Steal My Classmate?s Lover
• Curing Incurable Disease With Semen
• My Elder Sister Fell in Love With Me and Transformed Into a Yandere
• Nobunaga's Imouto is My Wife
• Status Meister
• The Ability to Make Woman be in Heat and Have Sex With Them Freely

well, some of them have incest, rape, netorare and other fetishes that not everyone likes, also i suggest searching on NU since they have a novel search feature that you can choose age rating (G, R-15, R-18 etc)

edit: forgot to mention, most of these novels have a very slow release time from their translators

stupid photobucket, wanting $400/yr just to display your images