the present tense is fine: está. What you are saying here is that you needed a book where this topic is explained well. If it is present tense in English then you can be confident is guessing that it is present tense in Spanish.

You can use the present tense (esté) of subjuntivo mode, (because in spanish there are modes) or imperfect tense of subjunctive (estuviera/ estuviese). For me sounds better the second one.

necesitaba means in the past and now is over necesito... it is now if you need information about SER Y ESTAR visit: http://www.studyspanish.com/lessons/serest1.htm Necesitaba un libro dónde ese tema (estar) bien explicado.... Necesito un libro donde expliquen el verbo ESTAR (this means now, you are looking for it) Necesitaba un libro donde expliquen el verbo ESTAR ( You know it now, you don't need it anymore) Hope this help you :-)