A College Companion by Jeanne Neumann provides a running outline/commentary on the Latin grammar covered in Book 1 of Lingua Latina (Familia Romana), and includes the complete text of the Ørberg ancillaries Grammatica Latina and Latin-English Vocabulary. It also replaces the student guide, Latine Disco. The book is designed especially for college students who approach Lingua Latina at an accelerated pace. The Exercitia may still be used as an additional source of exercises.
I book to use for grammar explanations alongside Familia Romana. I thought it'd be more useful than the Latīnē Discō, but there was not so much difference. There are some parts of it were the extra material in this book comes handy, but I'm the end I used LD more
The College Companion is a useful addition to the LLPSI main text. It lacks an index, though, which is crucial. I found myself often turning to Allen and Greenough or Gildersleeve when I couldn't locate a particular point of grammar I knew I'd come across before. Unfortunately, the Latin contained in the Companion should have been more carefully proofread; there are quite a few errors, particularly where macrons are concerned.