I am not sure what your level is, but I think you will find that material on accelerator structures generally requires upper division E&M. However in terms of concepts, this overview and these pictures (from Illinois Tech) will go a long way toward answering the questions you have lined out.
- In a drift-tube linac, the drift tubes sit inside a larger RF cavity and act as shields. Thus, the electric field inside a drift-tube can be approximated as zero. The gaps between drift tubes will still have an E-field, however, which varies sinusoidally in the direction of travel. When the particle is in the gap between drift tubes, it will experience an E-field and undergo acceleration.
- Let's say we are accelerating protons. We want to accelerate them in a particular direction, but half the time, the E-field in the gap is pointing in the wrong direction. Hence we can't make a continuous beam with this particular technology; we have to break the beam up into "bunches". This is by-and-large a limitation of all RF accelerating structures.
A truly comprehensive reference for Linacs is the SLAC "Blue Book", which is now freely available from SLAC. It is somewhat dated, but covers all aspects of building a linear accelerator in great detail. There are also a good number of SLAC publications that deal with Linacs, such as SLAC-PUB-7802. You may also consult material presented by the US Particle Accelerator School. Finally, there is the RF Linac textbook by Wangler.