I have found that a good introductory level book, comprehensible for upper-level undergraduates, is
"Introduction to Finite Elements in Engineering"
by Chandrupatla and Belegundu
I am familiar with some of the other books suggested, and they are also good. However, many of them are intended for, and written at, a graduate level. This particular book has been very useful for me in teaching the basic concepts to new engineers with no previous FEA background. When you say "very preliminary", this book is the first that comes to my mind. It does not require a continuum mechanics background to understand, and the notation and presentation are not as threatening as other texts.
Don't expect to read this book and have an understanding of advanced concepts--it is limited to linear problems.
If I could only have one FEA book, it would not be this. However, if I were teaching an undergrad FEA course, this is definitely the book I would use.
Brad