Since you have indicated in your comment that you are not only looking for textbooks, but any resource, I will provide a road map of resources of any form.
Disclaimer: I will not provide my opinions or a rating on the resources that I list, but solely how they work and for what level they are meant for. This is to avoid an opinion based answer.
This is split into levels and media forms in which the resources are available. Some resources may cover multiple levels and media forms.
Resources to have at any level:
These are resources that are extremely useful to have at the beginning of your learning and you will most likely use them throughout your French learning journey, even at advanced levels.
- French-English Dictionary. This is used to translate between the two languages, which is very useful in the learning process. Larousse and Collins Robert are two standard names to consider.
- Bescherelle : Conjugaison. Available in French and English versions, this is a must. It gives conjugations of verbs in every tense and mode in the French language. It also provides additional explanations on the nuances of certain verbs.
- Une grammaire. Collection of rules on French grammar. Bescherelle has one of these as well. Another must, especially at intermediate levels.
- A French dictionary. A dictionary solely in French with definitions and usages of the words. Larousse is a standard name to consider.
- Word Reference Online French English Dictionary. Online French English Dictionary for faster searches. This does not mean that you should shirk your paper dictionary, as it is important to be able to use both, and not become dependent on one or the other.
- Google Translate. Here I will offer a bit of caution. Read this discussion on the efficacy of Google Translate before using.
- Verbix French Verb Conjugator. Much like the Bescherelle Conjugaison, but an online resource. It is reliable for French but does not provide explanations like the Bescherelle does.
Beginner to Intermediate Level
Beginner to Advanced Level
These resources can be used at any level to improve French.
- The French for Dummies series. Takes an analytic approach to learning French.
- French Stack Exchange. Use this website to ask your questions, or find answers to questions that have already been asked.
- Duolingo. This is a free website used for language learning. It uses a mixture of approaches involving audio, typing, and speech analysis (depending on your platform). It can be used on iOS via the AppStore. It claims to be more effective than university education based on an independent scientific study. I will leave that judgment up to you.
- French.about.com. Maintained by Laura Lawless. It provides extensive explanations on many topics that French language learners encounter in their travels. It also has free learning resources such as audio exercises and quizzes.
- Rosetta Stone. This is software that teaches language completely using immersion. You won’t find any English in the courses. It uses audio, speech analysis, reading, and writing to achieve this. Note: This may appeal to you because of the fact that you started learning French at a young age, when language is often taught using immersion.
- Michel Thomas Method. A translation based approach for learning practical and conversational skills with the language. No writing or reading skills are covered.
- Pimsleur approach.
Intermediate to Advanced Level:
- Guide de la communication écrite. Written for use in CEGEP in Quebec, but acts as a very comprehensive textbook for referencing rules for written expression.
You may also consider taking courses from an academic institution. This can be helpful at any level.
A word of caution: Certain resources will claim to be able to teach you the language extremely fast, or in a certain amount of time. Remember that everyone learns at their own pace. Learning a language is an ongoing process and can take many years to master.
Patrick Sebastien’s answer is complete, but I want to add a textbook series and a piece of software that helped me learn French: