As far as a series goes, here is a good one.
Author: Laurell K. Hamilton
Referred to as "The Anita Blake Vampire Hunter" novels. There are 19 books in the series in paperback. The basic concept is a first person narration done by the main/title character, Anita Blake. It is set in modern times with a twist. It is known that vampires and shapeshifters (wereanimals) exist and the U.S. govt. has given them legal rights as citizens. Anita works at a company called Animators,Inc. Basically, she is an animator / necromancer that raises zombies for various reasons (family disputes inheritance; witness to crime died before testifying) and works as a preternatural crime expert for the police. Its set in St. Louis. In the 1st book. "Guilty Pleasures" the basic premise is set up that she becomes involved with a master vampire, JeanClaude, and the leader of the local werewolf pack,Richard.
The book is a lot more mature than "Twilight". Anita is a 20something yr old woman dealing with her job, love life, personal problems,etc. just like we all do except with a lot of twists.
Some of the books have more romance than others. And I have read all but 3 of them in the series and so far, only 2 had weak plots. Most of the time I devour them. I have an Associates in English and am working on my Early Childhood Development degree to be a K-6 teacher , so I am not the smartest person in the world by ANY means, but not the dumbest either and I have no problem following the plot, vocab,etc.
They really keep you wondering whats next; why did that happen;,etc.
I would classify them as adult horror/romance. They do not have near enough romance in them to be a stand alone romance, but enough is in it to spice it up.
The writing is superb, the characters fleshed out, and the back ground characters are awesome. You meet zombies; vamps; werewolves,wereleopards,wererats; faeries; regular human serial killers; voodoo priestesses and lots more.
Other good authors along the same genre are LA Banks and the Neteru Vampire Huntress series and Sherrilyn Kenyon. Kenyon, however is found in the "romance" section. Also, as far as werewolf and vamp go, another good one is Keri Arthur and the Riley Jensen series. Riley and her twin brother Roan are each half vamp/half werewolf and work for a preternatural version of the governments FBI. I would recommend all of them.
I know its hard to find a good book these days - hope you like them.
Laurell K.Hamilton's website actually has a part you can read the 1st and 2nd chapters for every book to decide whether you like them or not. Not sure the exact name; just type Laurell K.Hamilton into your search engine. She also writes another series about a female faerie private detective, but I have not read any of those.
GOOD LUCK