Particulary interested in the 'oo' prior to nasals, but also before 'l,' 'k,' and 'r.' Please help understand the linguistic ramifications of the differences, rules, phonological considerations, etc. I am writing a term paper for Professor Tubin in Beersheba.
I'm not sure what you mean by "linguistic ramifications," so any answer I give is going to be quite limited. One thing I can do is provide some information based on a database of 129,090 words, ranging from the most frequent to rare and recent adoptions. The string <oo> occurs in 2,919 of those words, though not always as the vowel digraph <oo>, for in several it is due to a concatenation, as in cooperate, coordinate, microorganism, epizoon. The digraph <oo> is the most important single spelling of long <u>, [u], as in, say, doom. Most of the older instances of <oo> stem from spellings of an earlier long <o>, which was often spelled with a double <o>, much as we today use double <e> to spell long <e>. Over time the long <o> was raised in the mouth, becoming long <u>. Thus, today <oo> most often spells long <u>, as in the following:
The string <oon> occurs in 307 words and consistently spells the sound [un], in the full range of words, from native words like boon and soon to recent adoptions like picaroon and tycoon.
The string <oom> occurs in 207 words, nearly always spelling [um]: bloom, room, gloom, etc. Holdouts are concatenations as in zoometry and the recent adoption woomera.
The string <ool> occurs in 240 words, nearly always spelling [u l]: fool, school, drool, etc. Among the few holdouts are the concatenations in <zool...> words, zoology, etc. and in words with the disyllabic Greek element oo meaning "egg": oogenesis, oolemma, etc.
In several, not always predictable cases, earlier [u] shortened to one of the modern short <u> sounds. Usually, the shortening was to , especially before <k>. (But notice the shortening to [u] in blood, flood.):
The string <ook> occurs in 303 words, nearly always spelling the high short <u>,[k] as in book or hook. In a handful of new words <ook> spells [u]: kook, spook, palooka.
The string <oor> occurs in 134 words. As usual, the pronunciation of vowels preceding [r] is marked with much variation, but <oor> spells something close to [or] in door and floor. And elsewhere it usually spells [r], as in poor, moor, hooray, etc. One holdout is whippoorwill, in which it spells the unstressed [r].
Good sources of further information are E. J. Dobson's English Pronunciation, 1500-1700, Eilert Ekwall's A History of Modern English Sounds and Morphology, and the first volume of Jespersen's A Modern English Grammar. I hope this helps.