Merge pull request #24 from MLSDev/TeTiRoss-patch-1

update backend recommended books section

Author: realsugar

Diff for: platform/backend/backend.md

@@ -302,3 +302,5 @@ coverage/
 
 * [Practical Object-Oriented Design in Ruby (by Sandi Metz)](https://www.amazon.co.uk/Practical-Object-Oriented-Design-Ruby-Addison-Wesley/dp/0321721330)
 * [Eloquent Ruby (by Russ Olsen)](https://www.amazon.co.uk/Eloquent-Ruby-Addison-Wesley-Professional/dp/0321584104/ref=pd_bxgy_14_img_2?ie=UTF8&refRID=HQ47S5P3BSG69ASDJH9A)
+* [Ruby Under a Microscope (by Pat Shaughnessy)](https://www.amazon.com/Ruby-Under-Microscope-Illustrated-Internals/dp/1593275277)
+* [Metaprogramming Ruby (by Paolo Perrotta)](https://www.amazon.com/Metaprogramming-Ruby-Program-Like-Facets/dp/1941222129)