Something like Amanda Stevens Graveyard series

[firewing]

A good adult series by Rick Riordan [who writes the Percy Jackson series for YA] is Jackson Navarre. The ghost was his partner from Texas Ranger days, who was killed [I think] on a raid into Mexico. The first book is called The Big Red Tequila.

Judy Clemens has the Grim Reaper series. Her character has Death for a ghost/familiar that follows her around.

Alice Kimberly has a series The Ghost and..., which is fairly lightweight [rather a cozy mystery series] but delightful. The character owns a bookstore, and her ghost is an old-time PI, who has always been confined to the bookstore itself, where he was killed. Lots of fun!

You might like the Odd Thomas series by Dean Koontz; Miss Peregrine's Home for Peculiar Children; Lily's Ghosts by Laura Ruby; Neil Gaiman's The Graveyard Book; Anna Dressed in Blood by Kendare Blake; Meg Cabot's Mediator series; Ghost Huntress series by Marley Gibson; Hear the Dead Cry & Dead Connection by Charlie Price; Between by Jessica Warman; Lily Dale series by Wendy Corsi Staub ; Walk of the Spirits by Richie Tankersley Cusick; Shadowed Summer by Saundra Mitchell; Give up the Ghost by Megan Krewe; Nearly Departed by Rook Hastings; or Possessed by Kate Cann. I highly recommend Apparitions by Raven Bower, anything by John Harwood or Jonathan Aycliffe, and the Ash series by James Herbert for top notch, intelligent ghost stories. For necromancers try the Black Wings series by Christina Henry or The Darkest Powers trilogy by Kelley Armstrong.

Felix Castor by Mike Carey : An exorcist that sees ghosts and try to get rid of them.
Necroscope by Brian Lumley : I couldn't get into the series after the 1st book, but you might like it.

And I second Odd Thomas.

Charley Davidson series by Darynda Jones. Charley is a Grim Reaper that sees ghosts and helps solve their murder. Is a fun read with lots of puns and jokes thrown in with the seriousness of the situation that Charley finds herself in.

Experiment in Terror by Karina Halle is another series with the main characters that put themselfs in weird situations where they can film the ghosts they encounter and is very thrilling.