A good basic theory book is about 250 pages, how you can use that information, is in the pages of thousands and thousands of books through the ages. Learning theory is easy, its the applications of it that take the time because they really work best when experience is involved.
To much practice can be a strain, so practice for 40mins, have a rest and read some basic theory while you have a drink and something to eat. This way you learn as you go along and can have some relevance and perspective to your level of playing.
For example, if you cannot name the notes you play, then read about how scales are constructed, then apply those to the fretboard and name the notes as you go. As well as helping you cover the full range of the fretboard, a really good work out for the hands, ( and brain as it will get you thinking and let you picture notes, scales, chords, intervals etc without having an instrument in your hands ) so it will give you a grounding on the next part of any theory to follow if you so wish.
If you want you could get one of the small theory books that deal with a chapter, then test you on it with questions. The great thing about theory is it is theory....no instrument required, you can do it all with a pen and paper, and for an instrument like bass it is best done as notation first then apply it to the bass.
A bass is not the easiest of instrument to learn how to see and hear the relationship in music, but once you have learned the basics, it and many other things just fall into place....it's a leap of faith in as much as you what you learn may not apply right now, but it will at some point and you will be prepared to deal with it better when it does.
A good place to start is, Music Theory for Dummies, http://www.amazon.co.uk/Music-Theory...ref=pd_sim_b_4
I downloaded it last week because I have a student that is using it, and I was impressed by his knowledge for only using it for a short time.
I was put off by the title, but it is a great book in that it it explains and sets up ideas really well using observations and other facts to support the ideas it teaches.
Because it is so well structured you could actually forget that you have to stop practice the ideas and learn them on an instrument.....and that is the beauty of such books....they can be applied to every instrument ever made because they deal with music, not any instruments perceived limitation, but music......it's the same 12 notes that Beethoven, Brahms, Mozart etc. used, the same 12 notes that Blues, Rock, Metal, Country, Reggae, Jazz etc have all used those same 12 notes.
It's funny when people think it has to do with the amount of strings.
Nothing could be further from the truth because its the same 12 notes.....just different or varying pitches involved. Funny that players like Miles Davis or Louis Armstrong ( both trumpeters ) had access to roughly the same amount of notes as a bass player ( around about three octaves ) but because they played their notes in a higher key they were not looked down on....Un-like a bass player......"must be easier, it has less strings".........."but still the same notes", as I like to remind anyone that aims that point at me.