This book won't make you an expert Latinist, but it is most useful for anyone who desires a reasonable understanding of the language, particularily if you are interested in understanding the Latin used in the liturgy. Readings from the Gospel of St. John are introduced in later lessons (the second book focuses more on the use of Latin in theology and cannon law). This first book does a decent job of covering Latin grammar completely; although, the explanations of grammatical constructs are a bit brief. It was written in 1944 and so presumes that the reader has had some exposure the the Latin Mass, but it won't be much of a hinderance if one hasn't had any exposure to the Latin Mass.
Overall, this is a wonderful introduction to the Latin used by the Church, and I heartily recommend it to anyone.
Collins slowly introduces you to Latin grammar while building up your Latin vocabulary. Scanlon & Scanlon tend to push you through the grammar more quickly, relying on cognates early on to help you cope with the vocabulary. Scanlon & Scanlon supply accent marks in all their texts and vocabularies to show you which syllable to accent; this is very helpful if you are teaching yourself. Collins provides macrons to mark long vowels in his texts and vocabularies; these suffice to let you figure out for yourself where the accent is, and they help you appreciate the meters used in classical poetry. For those who care about such things, physically Collins is clearly superior, with larger pages and larger type and plentiful margins. Scanlon & Scanlon comes with a splendid vocabulary in the appendix, small but complete, for reading the Breviary, the Missal, and the Gospels.
My approach to teaching myself Latin is "Teach me the grammar, give me a dictionary, and let me at the text". So I much prefer Scanlon & Scanlon to Collins. If you want to take the grammar more slowly though, you'll probably prefer Collins.
Honorable mention goes to Gavin Betts who wrote an excellent textbook, "Latin: A Complete Course (Teach Yourself)", that is intended for self-teaching. Unfortunately, its emphasis is almost entirely classical. Still, Classical and Ecclesiastical Latin are similar enough that it makes a good supplement to either Scanlon & Scanlon or Collins.
If you choose Collins, once you start reading the Vulgate, you'll probably want Leo F. Stelten's "Dictionary of Ecclesiastical Latin". Neither the Cassel's nor Chambers-Murray nor even the Oxford Latin Dictionary is adequate, although Lewis and Short's is, if you can afford it.