NavList:

I recently discovered a good manual for the Mark II sextant.  It's "Basic Optics and Optical Instruments" by the Bureau of Naval Personnel.  The version I have was republished by Dover Books in 1969 as an unabridged version of the 1966 Navy training course for Opticalman 3 & 2. (That would be 3rd and 2nd class petty officers.)  It's a 480 page paperback book.  The first half covers basic optics, which I ignored.  The rest covers general maintenance procedures, and specific details on a wide range of optical instruments, including binoculars, telescopes, gunsights, stadimeters, alidades, and of course, sextants.  The sextant chapter is 33 pages long and is specific to the Mark II sextant.  Lots of diagrams and specific disassembly/assembly instructions.

The book is out of print but there are a number of them for sale on eBay and Amazon.  Prices vary quite a bit, but I got one for under $10, including shipping.  One caution, though.  The training materials have been updated and newer versions are available.  In 1966 most of the sextants and other optical equipment used by the US Navy was still WWII vintage.  This stuff started being replaced in the 1970's.  So while the 1960's version covers the Mark II sextant, newer versions may cover newer sextants.