Apart from books, your most important source of information as a musician will be recordings. Listen to albums which feature harp players. Some important Blues players to look out for are: Sonny Terry, Little Walter, Sonny Boy Williamson, Junior Wells, Big Walter Horton, James Cotton, Billy Branch, Paul Butterfield, Snooky Pryor, Jerry Portnoy, Sugar Blue, Charlie Musselwhite and Rod Piazza.
For Country, Folk and Rock playing, listen to Charlie McCoy, Brendan Power, as well as the simple but effective playing of Neil Young and Bob Dylan. Some of the best Chromatic harmonica players include Larry Adler, Toots Thielemans and Stevie Wonder.
There are also numerous great Jazz and Blues sax players who are worth checking out. Little Walter got a lot of his ideas from listening to sax players. Some of the most Bluesy sax players are: Maceo Parker and Pee Wee Ellis (Solo or with James Brown) King Curtis, Junior Walker, Fathead Newman, A.c. Reed, Eddie Shaw, Eddie "Cleanhead" Vinson, Scott Page, Illinois Jacquet, Stanley Turrentine, Eddie Harris, Ben Webster, Johnny Hodges, and Roland Kirk who often played two saxophones at a time!
Guitar Players are another good source of ideas. Listen to the guitarist on any Blues album and you will hear note bending, slides, grace notes and other techniques which are equally effective on the harmonica. Some guitarists to look out for are BB King, Otis Rush, Buddy Guy (with Junior Wells or solo), Magic Sam, Lightnin’ Hopkins and Albert Collins along with Robert Junior Lockwood and Luther Tucker who can both be found on albums by Sonny Boy Williamson.
When you are listening to albums, try to sing along with the solos and visualize which holes you would play and the techniques you would use to achieve the sounds you are hearing. This helps you absorb the music and before long, it starts to come out in your own playing. It is also valuable to play along with albums, sometimes imitating what you are hearing and other times improvising. This is very good ear training and is also a lot of fun.