Hmmm... I guess I should try to say something in self-defense, beyond the reference I made to Jacobson's 1962 book *Lie Algebras* (since republished in paperback by Dover). It's conventional in many algebraic classifications to start with a *splitting field*, then descend to smaller fields by something like Galois cohomology. In the case of semisimple Lie algebras, which emerged from Elie Cartan's work a century ago on Lie groups and differential geometry, most people simplify by working first over an algebraically closed field of characteristic 0 (often specified arbitrarily as $\mathbb{C}$). Classically the objective was to understand simple Lie algebras (and Lie groups) over $\mathbb{R}$, but this gets quite complicated even though the field extension involved is quadratic.

The notion of splitting field here is fairly straightforward: you need all eigenvalues of ad-operators to be in your field in order to get a clean structure and classification theory. This mostly comes down to looking at Cartan subalegras (or "maximal toral subalgebras") in semisimple Lie algebras. But it's easier just to think about a familiar large field. Here the classification is relatively elementary, thanks to Killing and Cartan and their successors. But working over an arbitrary ground field gets quite intricate. Here some motivation comes from the parallel but more difficult study of semisimple algebraic groups.

Books and lecture notes abound, but usually at a fairly high level, and some books like those in the defunct Dekker series are hard to find as well as being photocopied from typescript. Reviews I wrote for the AMS Bulletin long ago of a couple of these are free online from AMS at emath.org, written by Goto-Grosshans and by my graduate advisor George Seligman from a different viewpoint.
I didn't mention in my various reference lists the standard old book by Helgason now republished by AMS: *Differential Geometry, Lie Groups, and Symmetric Spaces*. These and more recent books are pretty formidable, as are the much older research papers on structure and classification. It's all there, but are there readable and accessible modern references? Good luck.