I think you will find most books are knife-specific. I have a few that focus on lock backs.

Maybe someone else knows info on friction folders.

Friction folders are a very logical starting point.  There isn't a huge amount of info in the form of books on them... because they are fairly simple.  The biggest concern is to make things flat and parallel.  You can do something like splitting a handle, to building liners, etc.

  If your intent is go move into full fledged folders... such as lock backs or liner locks, there there are tons of reference material out there. 

  I personally only build liner locks, for a number or reasons.  The absolute best reference book I have ever used for liner locks is.... The Tactical Folding Knife, by Bob Terzoula.  I just looked on Amazon and holy moly!!  They want $150!  Whew!  However, it's still the best reference material I can recommend.  The price tag will prepare you for what's coming when you purchase folder building parts and tooling.  I can remember my first order for "folder making stuff".... the mailman dropped off a little 3x5" envelope.... that cost me $300!   And that still happens to this day. 🙂   If/When you get into folder in earnest, using only the best hardware is a must!  If you try to jack around with cheap China made stuff, all you will do is fight with it, and be frustrated.  Without peer, the best folder hardware produced today, is https://ticonnector.com/   

  The only book I would warn against is  How to Make Folding Knives: A Step-By-Step How-To.  While the ones who wrote this book are knife legends, I found it to be far too confusing, and often times misleading.  An example is that many passages infer that you must use a specific type/brand of tool, or the explained operation will not work. Looking back now, with my current knowledge level, its not an issue, but as a beginner, when I purchased this book, it caused me a lot of frustration, and made me think I would never be capable of building a folder.     

  OK...here's the best information I can offer to anyone making the jump into building folders....  Typically, when building straight knives, most think in terms of fractions of an inch, which is usually just fine. However, when building folders, you will need to be more precise.... and train yourself to think in fractions of a thousandths. That will only come through awareness and practice.  But once you get there, you will realize a notable improvement in all the knife you produce. 😉       

I would suggest that you look no further than this very forum. In the ABS University section, there is a whole sub-forum devoted to folding knives: Folding Knives – American Bladesmith Society Forum

The posts there cover friction folders and slip joints mostly, so for a beginner, this is where you might want to look. The tutorial by MS Steve Culver on page 2 of the sub-forum is especially well done. That will save you a lot of $ upfront. 

A long time ago, Tim Hancock gave me this book: https://www.amazon.com/Make-Folding-Knives-Step-Step/dp/0873413903 and it served me very well. 

If you give any of these a try, please post your efforts in the Show & Tell forum. It's a great way to get real time feedback at every stage of the build.

The new version of the Terzuola book is $40.  https://www.terzuola.net/product-page/terzuola-book-expanded-version