I absolutely despise reading books where every other word is technical jargon, and so I've come to you all for some advice.

Are there any grammar books out there, that aren't impossible to read, that would focus not on the all encompassing rules, but the less major ones?

The only thing I have at my disposal as of right now, is The Elements of Style (Which I've actually really enjoyed)

On another quick note... Are there any useful vocabulary building books out there? My vocabulary is probably average, but I'd really like to build it a bit.

A sampling;
ACS Style Guide
The Associated Press Stylebook
The Chicago Manual of Style
The Council of Biology Editors style manual
Turabian
The Elements of Typographic Style
ISO 690
MHRA Style Guide
The Little Brown Handbook
The MLA Handbook
The MLA Style Manual
The New York Times Manual
The Oxford Guide to Style
New Hart's Rules
The Publication Manual of the APA
Wikipedia's Style Manual
Words Into Type

For creative prose manuscript preparation, Chicago is most common or Words Into Type in some of the fantastical genres.

But it only deals with the highlights, and recently I decided I needed something heavier, but still accessible. I chose Michael Swan's "Practical English Usage", and it's primarily British English, me being a Brit 'n all. It's got more depth than the Economist's guide, yet still mostly uses plain English. The technical terms it uses are defined in a glossary at the front, so you don't have to use the main, more complex entries to understand its guidance. E.g. "verb" gets a few lines of definition in the glossary, and several pages of guidance in the main text.

For vocabulary building, I've just replaced J J Rodale's "Synonym Finder" with an Oxford Thesaurus. The Synonym Finder is American English (it doesn't know what a 'lorry' is).

The Elements of Style -Strunk and White
A Dash of Style -Noah Lukeman
Self Editing for Fiction Writers -Brown and King

These are not grammer books per se. My observation is that for writing fiction, it's more about understanding the options available and how to use them to good effect, and within the established conventions of prose fiction. Also, punctuation is employed somewhat differently in fiction than in other prose styles, making resources geared towards fiction writing more useful.

However, I would recommend an alternative approach to reading a book about grammar. There are many grammar practice tests available on the Internet. I have found that taking these tests, making mistakes, and seeing the mistakes has been the most effective way for me to learn correct grammar. My suggestion is to practice to near perfection.

Dee, I appreciate all the mentions. I do have the elements of style, and find it incredibly useful. I'll take a look at the other two you mentioned as well.

Philo, thanks for that idea. I hadn't thought of it, but yeah I'm sure there are tons of sources online. I'll have to see if there are any more specific type of tests I can take.

There's differences in recommendations, too. Serial commas, downstyle punctuation, and downstyle capitalization recommendations differ from one style to another. Chicago recommends ellipsis points to indicate faltering speech, Words, M dashes.

Numbers are handled differently across the board. Chicago is a more formal one, numbers up to but not including 101 are spelled out, higher numbers when they comprise no more than two syllables, whereas others recommend only spelling out numbers up to and including ten. Measurements are usually numerals no matter which style. Chicago addresses the distinctions quite extensively.

Citation quote marks are another area of wide difference. Whether to use logical quotes or typesetters' quotes is a frustrating decision. In general, if terminal punctuation is part of a citation, it belongs inside the terminal quote marks. If not, outside. However, no style manual that I know of places terminal punctuation outside of text emphasized with quote marks.

Examples;
Did you say, "let's go to the ballpark"?
In his signal treatise on war, Herringbottom said, "At what price an innocent's life?"
The term "inviolate," while subject to interpretation, can generally be regarded as meaning sacrosanct in most circumstances.

However, Chicago recommends italics instead of quotes when a word is used as a word. Conversely, italics are not always available in online formating. Bracketing with underscores is the traditional method in that circumstance.
The term inviolate,
The term _inviolate_,
The latter example is complicated by the terminal underscore being inside the comma, as it should be, but looks awkward, so quote marks might be easier to interpret. That's the wiggle clause in most style manuals, clarity comes first.

Chicago is available online as well as in print and on disc. A free thirty-day trial evaluation access is offered; http://www.chicagomanualofstyle.org/access/trial.epl

For vocabulary skills, the admonishment to read, read deep, read widely, has served me best. I have all my references to hand when I read. My favorite is a dictionary of synonyms. Unlike a thesaurus, a dictionary of synomyms addresses the nuances of synonymous meaning. Take a word, gift for example, a page and a half of the distinctions of its synonyms.

One other reference I recommend is a dictionary of usage. Webster's or Fowler's are the two most commonly used. Webster's American English, Fowler's, British English. The interesting thing about them, or frustrating, depending on point of view, is they rarely, unequivocably dictate a particular usage. Prescriptive usages are expressed as well as descriptive usages. In deciding which is proper, I usually use prescriptive usages for formal purposes, and descriptive for informal, creative usages.

I love the "Economist Style Guide" on split infinitives: all it says is, "To never split an infinitive is quite easy." So do we do it or not? I'm inclined never to split an infinitive unless to so do would be to totally render the text incomprehensible. In other words, I allow myself to sometimes split infinitives when I like how it reads, or when I want to subversively break formal rules.

I have to say, it was great. It wasn't so much about grammar, which was OK by me. But it definitely helped me with finding some problems in my prose. My character in the newest story I'm writing, feels much more alive than characters in previous stories.

Thanks a bunch for that recomendation, Dee.