Having thoughts about something suitable to bring this over from 3.5e to 5e.
Oddly, the SRD and PH don't give a page count for written spells, so carrying over from older rules, I'm presuming 1 page per spell level.
New Skill Use: Encode Spellbook
Requirements: Artisan Tools (Calligraphy Supplies)
Usage: To create an encoded spellbook, the Wizard (or Ritual Caster: Wizard) must already have copied the spell into their master book. He can then combine the spell with another written text (using double the pages, time and cost of "Replacing the Book", see Wizard sidebar) to conceal it from Harvesters (and other casters). This is written as a complete tome and spells can't be added to the encoded book later.
The Int (Investigation) DC to identify an encoded spellbook is: 10+Int+Proficiency (Calligraphy) and suffers Disadvantage until identified as a spellbook. This is set when the book is written (and may be much lower than the wizard's current ability).
The author can always prepare spells from the book, others must succeed in an Arcana skill check (using the above DC) to decipher and copy the spells (failure means loss of the scribing materials).
Example 1: Arianne Racine, a 5th level wizard (Int: 18, +3 proficiency and trained in Calligraphy tools), decides it is time to write a travelling spellbook. Her current book holds 25 spell levels (8 x 1st, 4 x 2nd, 3 x 3rd) after finding and copying a scroll of fireball. After 50 hours of work and 500gp in materials, she completes her 50 page treatise "Home Cooking in the Coryani Empire: Farm Recipes of the Plebians."
With a DC of 17 (10+4+3) to recognize it as a spellbook, it is unlikely to be recognized if her room is ransacked while adventuring.
Example 2: Bobo the clown, a 1st level wizard/4th level rogue (Int: 12, not proficient in Calligraphy tools) writes a book called "The Emperor's joke book". His current book only holds 6 spells, so only takes 12 hours and 120gp in materials. With only a DC of 11 (10+1), not only is it easy to detect as a spellbook, Coryani officials are likely to take offence at the jokes and drop him in the nearest prison.