rdale and mikedeason are both correct about the plots. The color scheme is pretty standard; at least UCAR also uses it in the their plots, and the square=automated scheme is adapted from NCEP central analysis plotting convention. In a severe weather situation, red/blue plots help alert me to either (1) ongoing weather, (2) widespread areas of lift that might be occurring north of a front, (3) radiational fog which usually burns off later in the morning, or (4) low ceilings that might be associated with return flow and are sometimes co-located with a LLJ.
The 850 mb map is traditionally used to examine the upper portions of the moist tropical layer and to examine the low-level jet. It can also be used to get a handle on finding the 850 mb fronts and assessing frontal slope. The caveat here is that (1) sometimes in shallow return flow the 850 mb level might actually be embedded within the old stagnant polar air or within the cap, and (2) as you go further west, the elevation in AGL (above ground level) decreases and it eventually intersects the ground. Still though it is useful for getting a basic look at the air just off the ground and getting a handle on things before refining the analysis with soundings, VAD wind profiles, and profiler data.
That said, unless you are following a prescribed analysis regimen there are not specific things you are looking for in checklist fashion, as it all depends on the situation. I think the best thing to do is look for the items above (without spending a whole lot of time) then as you synthesize information from other products and develop a "three dimensional" idea of the situation you would go back to the 850 mb chart a second time, this time with more insight, and see how the chart fits in with what you've analyzed so far.
As far as which book is most useful, it really depends on what you know so far. If you know a bit about fronts, highs, and lows already and are not too apprehensive about upper-level charts, you might go with Severe Storm Forecasting as the Diagnosis chapter elaborates on taking the ingredient-based and pattern-based methods and developing a forecast approach for a given situation. For a simpler handle on things, the Weather Forecasting Handbook is a good place to start, as well as the forecasting chapter of Storm Chasing Handbook (though it is markedly shorter).
Tim