How to use

• Set a camera of your scene as the active one, so you're looking through it.
• Place the 3D cursor at some point that you wish to keep focus on, like the nose of a character, the tip of a gun, the bottom of a cliff etc.
• Activate the tool in Object mode via menu View > Navigation > Dolly Zoom & Truck Shift or from a hotkey that you set up (read below in the "Installation" section).
• Drag the mouse to adjust the camera, and hold Shift for finer adjustments.
• Press Enter to confirm or Esc to cancel.
• While the tool is on, press F1 to show a help dialog with all shortcut keys and functions that the tool can do.
  

With the dolly-zoom mode, this tool changes both the FOV angle of the camera and its location in space. You can keyframe these properties for use with animation.
With the truck-shift modes, this tool changes both the shift of the camera and its location in space. You can also keyframe these properties.

Manual type-in mode

Following the same behavior as Blender's built-in operations, while this tool is on and in a certain adjustment mode (dolly-zoom, truck shift or rotate shift), if you type any number you will enter the "type-in mode". 
In this mode you can write the exact value for the adjustment being made:

• Besides numbers, typing a period or comma will add a decimal place to the number, and pressing backspace will remove the last character written.
  
• The number being typed will use the same units as the tool, that is, either a focal-length in millimeters, or an FOV angle in degrees. This unit type can be toggled while the tool is on (to see the shortcut key for this, please consult the F1 menu while the tool is on).
• Finally, hitting Enter will confirm the adjustment with that typed-in value.
  

Installation

Download the appropriate zip file for your Blender version after purchase: there's one version for Blender 2.79 and another for 2.80+. 

After installing the script in the Add-ons section of your Blender preferences and enabling it, you'll find the tool in the View > Navigation menu of the 3D viewport, it's called "Dolly Zoom & Truck Shift".

You can add a hotkey to this tool by right-clicking its menu and choosing "Add Shortcut".
You can also set a hotkey manually in the Input section of your Blender preferences: the operator name is view3d.dollyzoom_truckshift and should be added to the 3D View > 3D View (Global) category.