Agree with various posters above that the current commercial textbooks tend to be a mess.

I'll observe that the Common Core standards for grade 8 do call for doing systems of two linear equations. From:

http://www.corestandards.org/Math/Content/8/introduction/

Students solve systems of two linear equations in two variables and relate the systems to pairs of lines in the plane; these intersect, are parallel, or are the same line. Students use linear equations, systems of linear equations, linear functions, and their understanding of slope of a line to analyze situations and solve problems.

which is presumably why your DD is encountering them in her 8th grade equivalent pre-algebra class.

I personally have inclinations towards the sort of theoretical approach Val recommended.

However, I'll mention another approach that I've seen work well, which is the cpm.org approach. I would describe the CPM approach as "Common Core done right". CPM Is a non-profit consortium, staffed by middle and high school math teachers, that developed, pre-CC, a math curriculum based on collaboration, communicating math ideas to others, and problem solving, with spaced-repetition/spiraling rather than drill-and-forget. CPM was able to do a quick-and-natural transition to CC alignment, rather than the sort of garbled hack jobs coming out from the commercial textbook publishers.

I'm not personally familiar with their "Course 3" textbook (which would tend to be used as the 3rd year of a middle school curriculum, that is, grade 8, prior to an Algebra class in H.S.), but looking at the table of contents:

http://cpm.org/cc3

Chapter 1: Problem Solving
Chapter 2: Simplifying with Variables
Chapter 3: Graphs and Equations
Chapter 4: Multiple Representations
Chapter 5: Systems of Equations
Chapter 6: Transformations and Similarity
Chapter 7: Slope and Association
Chapter 8: Exponents and Functions
Chapter 9: Angles and the Pythagorean Theorem
Chapter 10: Surface Area and Volume

it does appear to be following a sensible enough order of working with equations in one variable, then graphing, then making sure students can go between graphs/functions/tabular sorts of representations, and only then moving on to systems of equations: the sort of topic ordering various posters recommended.

However, CPM books can be challenging to use as a supplement for an individual student since they do expect (and are built around) students working in groups and bouncing ideas off each other, and explaining their work to others: it's discovery-style learning, based on problem-solving and collaboration. So a CPM book may not be much help to the original poster looking for a supplement and way to fill in gaps for a single student -- but in a situation with another kid or two to work together through the material, I'd look at CPM (and then supplement as the kids seemed ready with additional "pure math" discussions of fundamental principles).