Modal Improvisation
 

hey, I play guitar for a couple of years, but I don't know how to improvise when it comes to modal music.
People say we must follow a scale when improvising. But how can I be sure on which scale a chord is? There are chords that fit in many scales. For instance, I believe Cmaj7 could be either in C Ionian or C Lydian scale (not sure, don't blast me if i'm wrong!!!!)

Should I examine a music sheet to determine all scales I'm going to use before playing the song?

Also, would anyone have some recommendations on books covering music theory and improvisation?