De Maria assumes you've done your reading, so it's not a great place to start, but it's a worthwhile read none the less. (Out of print, but Amazon usually has used copies starting around $2.50) A wonderful analysis of the major themes in Johnson's moral instruction essays. The Cambridge Companion to Samuel Johnson ($23.99) is a very valuable introduction to Johnson's writings.
Another book in this vein which is also very worthwhile is Lawrence Lipking's "Samuel Johnson: The Life of an Author", $19.50. Contents include not just an obligatory chapter on his life, but also chapters on his poetry, his essays, the Dictionary, his politics, his Shakespeare, religion, views on women, his letters, and his critical reception.
When I read Boswell, who lionizes Johnson, it seemed like the most natural thing in the world for Johnson to produce all the great writing that he did.
When I then read Bate, the triumph of Johnson's writing seemed all the greater.
This is the most popular place to begin reading about Samuel Johnson - - much of what we know comes from this intimate portrait.
Boswell gives us a record of their days together, their conversations, and the letters they exchanged -- many of the most famous Johnson quotes come from this book.
It's a very readable, fascinating book, not at all arcane, and well worth your attention. Bate has the advantage of being more removed, and is able to see Johnson as more of a human being as a result.
In addition to dealing with Johnson's successes, Bate gives greater treatment to Johnson's failures and personal difficulties than Boswell does.
Samuel Johnson's Dictionary, .93, edited by Jack Lynch.
Drawn from Johnson's 1st edition (1755), Jack has included over 3,000 of Johnson's entries, along with the quotations Johnson supplied to give force to the meanings.