The Jonathan shooter book is fine, but maybe a bit light.
Would advise to reread your airlaw book (ifr parts) and radio nav+pbn. They use the mentioned documents such as pans 8168 as source material.
Another good reading material which covers radio nav theory is pooleys air pilot manual vol 5.
Pan ops 8168 etc. are not written as learning material, and I wouldn’t bother treating them as such. Look into them if something is unclear or if you need more info.