Any efficient text-based steganographic schemes? While there are sophisticated and efficient steganographic schemes with images as cover available, I am yet ignorant of the existence of any fairly efficient and secure schemes with texts as cover. This state of affairs is IMHO unsatisfying, since the communication channel may under circumstances only be able to transfer printable natural language texts, and that in rather small volume, due to constraints posed by the warden. 

To be concrete on the efficiency issue, I would desire that the ratio of stego bits to covertext bits be no less than, say, 1/50. For references to existing schemes, if any, or else novel basic ideas that could lead to the development of such a scheme (if possible  user-friendly and not too hard to implement), I should be very grateful.

[Addendum 1, edited] I have in the meantime a scheme named WORDLISTTEXTSTEGANOGRAPHY that should achieve a not large but IMHO practically nonetheless useful efficiency in the range of [0.5, 1.0] bit per word of natural language texts, requiring though that the user does some amount of trial and error work under the guidance of the software. (Use e.g. of an appropriate codebook could highly compress the input stegobit sequence.) The latest Version 2.1 of it, running on Python V.3.6.1, is available at my new home page: http://mok-kong-shen.de. See update notes in it for the differences to the earlier versions. (I regret in particular that in the earlier versions a sentence is missing in the explanation of how to do the examples, with the very unfavourable consequence that new users may fail to do the examples.)
My understanding is that the three most popular approaches to "steganography using text as a cover" are:

1 Generate a completely new text by picking one word at a time from a dictionary, using the ciphertext bits to select which word.


Pick words in ways that, at first glance, look like real English sentences, using Markov chain algorithms.
Weihui Dai.
"Text Steganography System Using Markov Chain Source Model and DES Algorithm".
(2010).
H. Hernan Moraldo.
"An Approach for Text Steganography Based on Markov Chains".
(2012).
"Steganographic Markov Chains".
(2008).
Anna Lysyanskaya and Mira Meyerovich
"Provably Secure Steganography with Imperfect Sampling"
(2006). Etc.
For a given dictionary, picking words with uniform distribution gives you the most ciphertext bits per word,
but it's pretty obviously not real English text. Still, these schemes are very popular for sending raw bits over systems (like email, voice, etc.) designed for text: NATO phonetic alphabet, PGP word list, the S/KEY dictionary, the Diceware dictionary, etc.


2 Take some fixed text, and send exactly the same series of letters as in the original text, adding the ciphertext bits by modifying the presentation of that text in ways that we hope the warden doesn't notice.


Use two slightly different typefaces as in Bacon's cipher (1605).
Add space characters to the end of lines, where they are usually invisible, as in SNOW (1996?).
Vary the spacing between words; nudge the letters left-right (nonstandard kerning) or up-down.
Etc.


3 Take some fixed text, and slightly modify the words in ways that we hope the warden doesn't notice.


The receiver takes each word, looks it up in the agreed-on thesaurus; if the thesaurus shows 4 synonyms, the receiver decodes that word into 2 ciphertext bits. Nanhe, Kunjir, Sakdeo. “Improved Synonym Approach to Linguistic Steganography”. Shirali-Shahreza. "A New Synonym Text Steganography". Chang and Clark. "Practical Linguistic Steganography using Contextual Synonym Substitution and Vertex Colour Coding".
The receiver looks for certain "key words", ignoring all the other words.
The receiver decodes a few bits per sentence by noticing which of several ways (all with equivalent meaning) of arranging the words in that sentence were selected. Grothoff; Grothoff; Alkhutova; Stutsman; and Atallah. "Translation-Based Steganography" (2005). a
Etc.