“Adjoin” is another Latin derivative, this time quite clearly from ad + iungere “toward join”.

• 09.037 The rest went into the adjoining cellar with the trapdoors.

“adjoin, v.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

I was interested to learn if “acquaint” was one of the English “a-” prefix or preposition words, but that letter A is of more classical ancestry, it’s the Latin ad of adcognitare.

• 06.030 who was acquainted with this part of the mountains;

“acquaint, v.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

This use of “a-” forms adverbs and prepositions and comes from the “on – ” preposition which we saw in “ablaze”, so both Prefix 3 and Preposition 1 are in play here!

• 01.109 and three may walk abreast” say the runes,
• 03.040 and three may walk abreast.'”
• 13.048 wide enough for many men abreast.

“a-, prefix3.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.

“a, prep.1.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“abreast, adv. and prep.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.

“Ablaze” enters The Hobbit in its adverbial form (it can also be an adjective), and only in Chapter 14 in reference to Smaug!  The “a-” prefix is the unstressed spelling of “on” as in “on fire” which became one word with use.

• 14.014 seeking only to set their town ablaze.
• 14.017 Soon he would set all the shoreland woods ablaze

“a, prep.1.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.

“ablaze, adj. and adv.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.

In the sense of “dwelling place”, OED  calls this a literary word, and I temporarily tagged it thus.  (Update 2015.06.08) Only one other literary word raised its head in our explorations, so I remove the tag.

Apparently “a-” here is a variant of the unstressed prefix “or-”, which creates nouns and verbs, as in “ordeal”.

18.032 There now Dain son of Nain took up his abode,

“a-, prefix1.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.

“abide, v.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.

“abode, n.1.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.

“† or-, prefix.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.