Reddit mentions of Mandolin Picker's Guide to Bluegrass Improvisation

There's not as formal of a progression with mandolin as there are with more established jazz/classical instruments. I think the closest that you're going to find would be a book like The Complete Mandolinist. It has a decidedly classical bent to it, but it's a single book but it was created to be used progressively rather than a random selection of tunes.

I've also heard good things about Horne's series. but they are well reviewed. More folk/bluegrassy oriented.

It doesn't answer your question, but depending on your level and what you're looking for, there are two books that I have that are more exercise oriented that focus on a progressive approach to learning imrpovisation and the fretboard-- Esichman's getting into Jazz Mandolin has years worth of exercises to develop a Jazz vocabulary and fretboard knowledge.

And Petersen's Guide to Bluegrass Improvisation focuses on building a bluegrass improv vocabulary.

Sorry I don't have much more help, but I don't know if there is a correct answer to this one.

Do you have prior experience with stringed instruments? If so I strongly suggest The Mandolin Picker's Guide To Bluegrass Improvisation, and Getting Into Jazz Mandolin, the don't require a lot of experience, but you might struggle if you have none at all. I'm sure some ones suggested Mandolin Cafe, but If you are a complete beginner, Folk of the Wood Is another good place to start.

The thing that strikes me about MM when listening to his instructional videos on YouTube is that when talking music (not posture, not technique, but notes) he talks like an improviser... which can be hard to follow if you don't already know lots of scales and chord progressions. "So in this lick you can throw in a C here, which gives it a little bluesy sound." "Well why the hell does it DO that, Mike?" So I've been a little reluctant to sign up for his courses. Awesome player, but communication-wise, might be better to learn from someone who has had to articulate all of this stuff for themselves because it didn't come naturally.

For theory: You may want a book. You could go for a straight-up music theory book, but for something geared towards mandolin, I think that the book by Horne and Fugate did an okay job on theory. Mike Marshall also publishes an "Improvisational Concepts" book for mandolin (look at Elderly.com for it, I think). It is a little sparse, but if you are willing to work with the exercises and design some of your own it can work. Pretty bluegrass specific, but I think this book is outstanding, lots of depth, slowly ratchets up your improv level.

For technique: Watch videos and mimic. Video yourself practicing (it helps!)

Learning random songs: There is a lot to be said for learning random songs as a tool for learning theory...as long as you are consciously connecting it to the theory, rather than just playing tab. I like the Jethro Burns mandolin book for this reason. It looks mostly like a big collection of songs...but Jethro is the absolute freaking master of licks, turnarounds, and finishes. If you spend some time figuring out how his licks work with the chord progressions and melody you will not be sorry.

I recently picked up a copy of this book and have been extremely pleased with the approachability for newcomers as well as the depth of lessons as you get more advanced http://www.amazon.com/presents-Mandolin-Pickers-Bluegrass-Improvisation/dp/078668237X

Gives you exercises to help you learn the scales and theory behind what you're playing as opposed to just a lead sheet or fakebook.

https://www.amazon.com/Mandolin-Pickers-Guide-Bluegrass-Improvisation/dp/078668237X

Do you want a theory book, go over pentatonics/3 minor scales, chords to learn after major/minor triads? The above is really good, but you can learn that from a lot of books in yoru library, this is another https://www.amazon.com/Guitar-Theory-Dummies-Online-Instruction/dp/1118646770/

The other, non theory way to go is to listen to lots of version of, say, Whiskey before breakfast, hear all the melody, harmonic and rhythmic variations and incorporate that into your playing without thinking "oh year, this picker really like mixolydian"