For reasons pertaining to "virtual inventory management", as well as the compatibility of each volume's subject matter - what was two, now is one.
For a full description of what have become individual chapters of the same volume, see the original blog posts for "Fourtitude" and "Lines in 4ths: iii-VI-ii-V7", which are still in tact, and will remain so.
Now as before, the "Fourtitude" section of the book focuses on exploring the uses of Perfect 4th pairs in creating melodic lines.. This concept occasionally overlaps with that of the recent Shortbooks™, "Trichord 027", "The Cycle - 101" and "Pentalogy", which all link the interval of a P4th and the Pentatonic Scale inherently.
Along with P4ths, the section on "Lines in 4ths" allows for Augmented 4ths (tritones), as well as diminished 4ths (sounds a Maj 3rd) in the creation of lines over the familiar iii-VI-ii-V7-I chord progression.