The nice thing about the transliteration is that it (1) allows you to start learning grammar and vocabulary while you're still trying to get a handle on the massive syllabary, (2) makes the book more accessible for comparative Semiticists, and (3) helps you see two important features that are not clearly communicated in the Ge'ez script: gemination (consonant doubling) and when the sixth order consonants are terminal vs. vocalized. The bad thing is . . . it's transliterated. The actual Ethiopic texts you will read are not transliterated, and learning the vocab and grammar in transliteration can make it pretty tough to switch mental gears when you begin reading texts.
That's why Osvaldo Raineri's Introduzione Alla Lingua Ge'ez, an Italian translation of Lambdin, is so useful. It presents the vocabulary and exercises in the Ge'ez script (and transliteration early in the book). This makes it an excellent, even necessary companion to English Lambdin. If you have both volumes you get the best of both worlds.
You can look for it in a library at WorldCat.You can find it used or new through BookFinder (ISBN is 8872103355).
I got mine new through Deastore.com for 24 euros. (Important: if you buy it here, be sure to go through the registration process in order to get free international shipping -- the quick PayPal purchasing process does not seem to give you the option for free shipping.