As Purana kagaz said if u have10 years experience then offcourse u r more knowledgeable than any PhD holder in that field and i guess u know that pretty much well. If u really want it to do from academic point of view then i will not say it is just a waste of time n money. Few things that i would recommend you are
1) build up strong mathematical concept ( basically calculus 'cause u gona messed up with it )
2) signals and system ( fourier series, fourier transform, z transform, DFT/FFT, LTI system, casaulity etc..)
3) Good concept of probability and statistics so u can deal well with random signal
4) filter design
The books i recommend are
- Discrete-Time Signal Processing by A. V. Oppenheim and R. W. Schafer ( i used it as textbook)
- Digital Signal Processing: Principles, Algorithms, and Applications by J. G. Proakis and D. G. Manolakis( good reference book)
From my personal view point B.P. lathi is easy to understand but it is so basic. U r going to have MSEE in signal processing so i recommend u to go through book #1