While nothing compares to practicing a language through conversations and interactions with it in a natural setting, it is important to recognize that it can be very difficult to do so extensively, whether it is due to availability or cost. Good resources and good autonomous studying practices combined with a reasonable amount of practices can go a long way.
Here is a list of resources that I have worked with in and out of my classrooms and that I have found to be effective, relevant, or that students have enjoyed.
English Grammar Students of French https://www.amazon.com/English-Grammar-Students-French-Learning/dp/0934034427 This one is very useful for English speakers. A lot of students struggle with grammar because they don't know its rules (even though they apply it every day!). This book is really helpful because it explains English grammar and compares it to its French equivalent. Bonus: you can find used older editions for a very low price!
ABC DELF A2 https://www.amazon.fr/ABC-DELF-Niveau-A2-Livre/dp/2090381728 I have use this one for its listening and reading exercices that are very well done. While it is geared towards the A2 level, I find a lot of them easy enough to be used in A1 as well. I don't like the one available for A1 as much, its scope is more limited et felt that it wouldn't take much time to complete.
Le Besherelle (conjugation) https://www.amazon.com/Bescherelle-Conjugaison-Pour-Tous-Reference/dp/2218951983/ref=sr_1_2?s=books&ie=UTF8&qid=1513268086&sr=1-2&keywords=besherelle A classic! One of the best references for verb conjugations. As a kid in France, that's the one I had. If you prefer online material, l'OBS conjugaison is great too! http://la-conjugaison.nouvelobs.com/
Alter Ego 3 (B1) and Edito B1 https://www.amazon.com/Alter-Ego-Methode-Francais-French/dp/2011555124/ref=sr_1_1?s=books&ie=UTF8&qid=1513268418&sr=1-1&keywords=alter+ego+3 https://www.amazon.com/Nouvel-Edito-Livre-LEleve-French/dp/2278072692/ref=sr_1_1?s=books&ie=UTF8&qid=1513268471&sr=1-1&keywords=french+edition+b1 For the intermediate levels, I like to mix these two up. What I like in Alter Ego 3 is the material used, a lot of cultural artifacts, as well as tips for writing. It is a fun book with relevant. However, it does lack vocabulary sections, which I still find necessary at that level. It is for that reason, that overall, I prefer Edito which I find a little more well-rounded as it includes a lot of artifacts, grammar explanations and vocabulary exercises.
http://www.wordreference.com/ This is my favorite online dictionary because it shows expressions using the search word, its usages as well as forums to ask questions about unclear usages.
http://www.bonjourdefrance.com/ This website offers a lot of self-correcting exercises for all levels and all language-skills. It is a great resource to practice! However, depending on who build the exercices you work on, you don't necessary have any explanations. So if you need help understanding a specific grammar point, it might not be the best. A must to use if you want to check that you know a specific aspect of the language!
https://www.podcastfrancaisfacile.com/ A lot of resources available here too. I have only used it for listening comprehesion so far, so I cannot say anything about the rest. But the listening ones are great. For a lot of students, listening comprehension is one of the hardest thing to master, and this website offers a lot of audio files with scripts so that you can really practice on your own.
Please leave a comment if you have material that you love to use!